Modus themes for GNU Emacs
**************************

This manual, written by Protesilaos Stavrou, describes the customisation options for the modus-operandi and modus-vivendi themes, and provides every other piece of information pertinent to them.

The documentation furnished herein corresponds to version 0.13.0, released on 2020-10-08.  Any reference to a newer feature which does not yet form part of the latest tagged commit, is explicitly marked as such.

Copyright (C) 2020 Free Software Foundation, Inc.

     Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.3 or any later version published by the Free Software Foundation; with no Invariant Sections, with no Front-Cover Texts, and with no Back-Cover Texts.

Overview
********

The Modus themes are designed for accessible readability.  They conform with the highest standard for colour contrast between any given combination of background and foreground values.  This corresponds to the WCAG AAA standard, which specifies a minimum rate of distance in relative luminance of 7:1.

Modus Operandi (modus-operandi) is a light theme, while Modus Vivendi (modus-vivendi) is dark.  Each theme's colour palette is designed to meet the needs of the numerous interfaces that are possible in the Emacs computing environment.

The overarching objective of this project is to always offer accessible colour combinations.  There shall never be a compromise on this principle.  If there arises an inescapable trade-off between readability and stylistic considerations, we will always opt for the former.

To ensure that users have a consistently accessible experience, the themes strive to achieve as close to full face coverage as possible (see Face coverage).

Starting with version 0.12.0 and onwards, the themes are built into GNU Emacs (current version is 0.13.0).

How do the themes look like
===========================

Check the web page with the screen shots (https://protesilaos.com/modus-themes-pictures/).  There are lots of scenaria on display that draw attention to details and important aspects in the design of the themes.  They also showcase the numerous customisation options.

Customisation options.

Learn about the latest changes
==============================

Please refer to the web page with the change log (https://protesilaos.com/modus-themes-changelog).  It is comprehensive and covers everything that goes into every tagged release of the themes.

Installation
************

The Modus themes are built into Emacs' development branch (master) which corresponds to the nominal version 28.0.50.  Otherwise they can be installed using Emacs' package manager or manually from their code repository.

Modus Operandi (light theme) and Modus Vivendi (dark) are normally distributed as standalone packages in Emacs-specific archives: GNU ELPA, MELPA, and MELPA Stable.  There also exist packages for GNU/Linux distributions.

Install from the archives
=========================

modus-operandi-theme and modus-vivendi-theme are available from GNU ELPA, MELPA, MELPA Stable.

Prior to querying any package archive, make sure to have updated the index, with M-x package-refresh-contents.  Then all you need to do is type M-x package-install and specify the theme of your choice.

GNU ELPA and MELPA Stable contain the last tagged release.  While MELPA builds from the latest commit to the Modus theme's Git repository (https://gitlab.com/protesilaos/modus-themes).

Install on GNU/Linux
====================

The themes are also available from the archives of some GNU/Linux distributions.  These should correspond to a tagged release rather than building directly from the latest Git commit.  It all depends on the distro's packaging policies.

Debian 11 Bullseye
------------------

The two themes are distributed as a single package for Debian and its derivatives.  Currently in the unstable and testing suites and should be available in time for Debian 11 Bullseye (next stable).

Get them with:

     sudo apt install elpa-modus-themes

GNU Guix
--------

Users of either the Guix System (the distro) or just Guix (the package manager) can get each theme as a standalone package.

     guix package -i modus-operandi-theme

And/or:

     guix package -i modus-vivendi-theme

Enable and load
***************

This section documents techniques on how to load the theme of your choice and how to further control its initialisation.  It also includes some sample code snippets that could help you in the task, especially if you intend to use both Modus Operandi and Modus Vivendi.

Load automatically
==================

A simple way to load the theme from your Emacs initialisation file is to include either of the following expressions:

     (load-theme 'modus-operandi t)          ; Light theme
     (load-theme 'modus-vivendi t)           ; Dark theme

Make sure to remove any other theme that is being loaded, otherwise you might run into unexpected issues.

Note that you can always M-x disable-theme and specify an item.  The command does exactly what its name suggests.  To deactivate all enabled themes at once, in case you have multiple of them enabled, you may evaluate the expression:

     (mapc #'disable-theme custom-enabled-themes)

Load at a given time or at sunset/sunrise
=========================================

It is possible to schedule a time during the day at or after which a given theme will be loaded. (Contributed on Reddit by user b3n https://www.reddit.com/r/emacs/comments/gdtqov/weekly_tipstricketc_thread/fq9186h/.)

     ;; Light for the day
     (load-theme 'modus-operandi t t)
     (run-at-time "05:00" (* 60 60 24)
                  (lambda ()
                    (enable-theme 'modus-operandi)))

     ;; Dark for the night
     (load-theme 'modus-vivendi t t)
     (run-at-time "21:00" (* 60 60 24)
                  (lambda ()
                    (enable-theme 'modus-vivendi)))

A modified version of the above technique is to use the sunrise and sunset as references, instead of specifying a fixed hour value. (Contributed directly by André Alexandre Gomes https://gitlab.com/aadcg.) If you set calendar-latitude and calendar-longitude (defined in the built-in solar.el library---read it with M-x find-library), you can automatically switch between both themes at the appropriate time-of-day. Note that those calendar variables need to be set before loading the themes.

     ;; Define coordinates
     (setq calendar-latitude 35.17
           calendar-longitude 33.36)

     ;; Light at sunrise
     (load-theme 'modus-operandi t t)
     (run-at-time (nth 1 (split-string (sunrise-sunset)))
                  (* 60 60 24)
                  (lambda ()
                    (enable-theme 'modus-operandi)))

     ;; Dark at sunset
     (load-theme 'modus-vivendi t t)
     (run-at-time (nth 4 (split-string (sunrise-sunset)))
                  (* 60 60 24)
                  (lambda ()
                    (enable-theme 'modus-vivendi)))

For the sake of completeness, the load-theme call in these snippets is slightly different than the one shown in Load automatically, because it does not enable the theme directly: the subsequent enable-theme does that when needed.

Toggle between the themes on demand
===================================

With both themes available, it is possible to design a simple command to switch between them on demand.

     (defun modus-themes-toggle ()
       "Toggle between `modus-operandi' and `modus-vivendi' themes."
       (interactive)
       (if (eq (car custom-enabled-themes) 'modus-operandi)
           (progn
             (disable-theme 'modus-operandi)
             (load-theme 'modus-vivendi t))
         (disable-theme 'modus-vivendi)
         (load-theme 'modus-operandi t)))

You could use (mapc #'disable-theme custom-enabled-themes) instead of disabling a single target, but you get the idea.

Configure options prior to loading
==================================

If you plan to use both themes and wish to apply styles consistently (see Customisation Options), you could define wrapper functions around the standard load-theme command.  These extend the simple function we presented in Toggle between the themes on demand.

Here is a comprehensive setup (the values assigned to the variables are just for the sake of this demonstration): (The defmacro and dolist method were contributed on Reddit by user b3n https://www.reddit.com/r/emacs/comments/gqsz8u/weekly_tipstricketc_thread/fsfakhg/.)

     (defmacro modus-themes-format-sexp (sexp &rest objects)
       `(eval (read (format ,(format "%S" sexp) ,@objects))))

     (dolist (theme '("operandi" "vivendi"))
       (modus-themes-format-sexp
        (defun modus-%1$s-theme-load ()
          (setq modus-%1$s-theme-slanted-constructs t
                modus-%1$s-theme-bold-constructs t
                modus-%1$s-theme-fringes 'subtle ; {nil,'subtle,'intense}
                modus-%1$s-theme-mode-line '3d ; {nil,'3d,'moody}
                modus-%1$s-theme-faint-syntax nil
                modus-%1$s-theme-intense-hl-line nil
                modus-%1$s-theme-intense-paren-match nil
                modus-%1$s-theme-no-link-underline t
                modus-%1$s-theme-no-mixed-fonts nil
                modus-%1$s-theme-prompts nil ; {nil,'subtle,'intense}
                modus-%1$s-theme-completions 'moderate ; {nil,'moderate,'opinionated}
                modus-%1$s-theme-diffs nil ; {nil,'desaturated,'fg-only}
                modus-%1$s-theme-org-blocks 'greyscale ; {nil,'greyscale,'rainbow}
                modus-%1$s-theme-headings  ; Read further below in the manual for this one
                '((1 . section)
                  (2 . line)
                  (t . rainbow-line-no-bold))
                modus-%1$s-theme-variable-pitch-headings nil
                modus-%1$s-theme-scale-headings t
                modus-%1$s-theme-scale-1 1.1
                modus-%1$s-theme-scale-2 1.15
                modus-%1$s-theme-scale-3 1.21
                modus-%1$s-theme-scale-4 1.27
                modus-%1$s-theme-scale-5 1.33)
          (load-theme 'modus-%1$s t))
        theme))

     (defun modus-themes-toggle ()
       "Toggle between `modus-operandi' and `modus-vivendi' themes."
       (interactive)
       (if (eq (car custom-enabled-themes) 'modus-operandi)
           (progn
             (disable-theme 'modus-operandi)
             (modus-vivendi-theme-load))
         (disable-theme 'modus-vivendi)
         (modus-operandi-theme-load)))

Customisation Options
*********************

The Modus themes are highly configurable.  Though they should work well without any further tweaks.

By default, all customisation options are set to nil.

All customisation options need to be evaluated before loading their theme (see Enable and load).

Option for more bold constructs
===============================

Symbol names:

  * modus-operandi-theme-bold-constructs

  * modus-vivendi-theme-bold-constructs

Possible values:

 1. nil (default)

 2. t

Display several constructs in bold weight.  This concerns keywords and other important aspects of code syntax.  It also affects certain mode line indicators and command-line prompts.

The default is to only use a bold weight when it is required.

Additionally, and while not necessary, to define the precise weight for bold constructs, you can change the typographic intensity of the bold face.  The standard is a bold weight.  It requires no further intervention.  Assuming though that your typeface of choice supports a ``semibold'' weight, adding the following snippet to your init file should suffice.

     (set-face-attribute 'bold nil :weight 'semibold)

Note that if you are switching themes, you need to re-evaluate this expression after the new theme is loaded.

Option for more slanted constructs
==================================

Symbol names:

  * modus-operandi-theme-slanted-constructs

  * modus-vivendi-theme-slanted-constructs

Possible values:

 1. nil (default)

 2. t

Choose to render more faces in slanted text (italics).  This typically affects documentation strings and code comments.

The default is to not use italics unless it is absolutely necessary.

Option for faint code syntax highlighting
=========================================

Symbol names:

  * modus-operandi-theme-faint-syntax

  * modus-vivendi-theme-faint-syntax

Possible values:

 1. nil (default)

 2. t

Use less saturated colours in programming modes for highlighting code syntax.  The default is to use saturated colours.

This option essentially affects the font-lock faces, so it may also have implications in other places that are hard-wired to rely directly on them instead of specifying their own faces (which could inherit from font-lock if that is the intent).  The author is aware of vc-dir as a case in point.

Option for no font mixing
=========================

Symbol names:

  * modus-operandi-theme-no-mixed-fonts

  * modus-vivendi-theme-no-mixed-fonts

Possible values:

 1. nil (default)

 2. t

By default, the themes configure some spacing-sensitive faces, such as Org tables and code blocks, to always inherit from the fixed-pitch face. This is to ensure that those constructs remain monospaced when users opt for something like the built-in M-x variable-pitch-mode.  Otherwise the layout would appear broken.  To disable this behaviour, set the option to t.

Users may prefer to use another package for handling mixed typeface configurations, rather than letting the theme do it, perhaps because a purpose-specific package has extra functionality.  Two possible options are org-variable-pitch (https://melpa.org/#/org-variable-pitch) and mixed-pitch (https://melpa.org/#/mixed-pitch).

Option for no link underline
============================

Symbol names:

  * modus-operandi-theme-no-link-underline

  * modus-vivendi-theme-no-link-underline

Possible values:

 1. nil (default)

 2. t

Remove the underline effect from links, symbolic links, and buttons. The default is to apply an underline.

Option for command prompt styles
================================

Symbol names:

  * modus-operandi-theme-prompts

  * modus-vivendi-theme-prompts

Possible values:

 1. nil (default)

 2. subtle

 3. intense

The symbols ``subtle'' and ``intense'' will apply a combination of accented background and foreground to the minibuffer and other REPL prompts (like M-x shell and M-x eshell).  The difference between the two is that the latter has a more pronounced/noticeable effect than the former.

The default is to not use any background for such prompts, while relying exclusively on an accented foreground colour.

Option for mode line presentation
=================================

Symbol names:

  * modus-operandi-theme-mode-line

  * modus-vivendi-theme-mode-line

Possible values:

 1. nil (default)

 2. 3d

 3. moody

The default value (nil) produces a two-dimensional effect both for the active and inactive modelines.  The differences between the two are limited to distinct shades of greyscale values, with the active being more intense than the inactive.

A 3d symbol will make the active modeline look like a three-dimensional rectangle.  Inactive modelines remain 2D, though they are slightly toned down relative to the default.  This aesthetic is the same as what you get when you run Emacs without any customisations (emacs -Q on the command line).

While moody removes all box effects from the modelines and applies underline and overline properties instead.  It also tones down a bit the inactive modelines.  This is meant to optimise things for use with the moody package (https://github.com/tarsius/moody) (hereinafter referred to as ``Moody''), though it can work fine even without it.

Note that Moody does not expose any faces that the themes could style directly.  Instead it re-purposes existing ones to render its tabs and ribbons.  As such, there may be cases where the contrast ratio falls below the 7:1 target that the themes conform with (WCAG AAA).  To hedge against this, we configure a fallback foreground for the moody option, which will come into effect when the background of the modeline changes to something less accessible, such as Moody ribbons (read the doc string of set-face-attribute, specifically :distant-foreground).  This fallback comes into effect when Emacs determines that the background and foreground of the given construct are too close to each other in terms of colour distance.  In effect, users would need to experiment with the variable face-near-same-color-threshold to trigger the fallback colour. We find that a value of 45000 would suffice, contrary to the default 30000.  Do not set the value too high, because that would have the adverse effect of always overriding the default colour (which has been carefully designed to be highly accessible).

Furthermore, because Moody expects an underline and overline instead of a box style, it is recommended you also include this in your setup:

     (setq x-underline-at-descent-line t)

Option for completion framework aesthetics
==========================================

Symbol names:

  * modus-operandi-theme-completions

  * modus-vivendi-theme-completions

Possible values:

 1. nil (default)

 2. moderate

 3. opinionated

This is a special option that has different effects depending on the completion UI.  The interfaces can be grouped in two categories, based on their default aesthetics: (i) those that only or mostly use foreground colours for their interaction model, and (ii) those that combine background and foreground values for some of their metaphors. The former category encompasses Icomplete, Ido, Selectrum as well as pattern matching styles like Orderless and Flx.  The latter covers Helm, Ivy, and similar.

A value of nil will respect the metaphors of each completion framework.

The symbol moderate will apply a combination of background and foreground that is fairly subtle.  For Icomplete and friends this constitutes a departure from their default aesthetics, however the difference is small.  While Helm et al will appear slightly different than their original looks, as they are toned down a bit.

The symbol opinionated will apply colour combinations that refashion the completion UI.  For the Icomplete camp this means that intense background and foreground combinations are used: in effect their looks emulate those of Ivy and co. in their original style.  Whereas the other group of packages will revert to an even more nuanced aesthetic with some additional changes to the choice of hues.

To appreciate the scope of this customisation option, you should spend some time with every one of the nil (default), moderate, and opinionated possibilities.

Option for fringe visibility
============================

Symbol names:

  * modus-operandi-theme-fringes

  * modus-vivendi-theme-fringes

Possible values:

 1. nil (default)

 2. subtle

 3. intense

The ``subtle'' symbol will apply a greyscale background that is visible, yet close enough to the main background colour.  While the ``intense'' symbol will use a more noticeable greyscale background.

The default is to use the same colour as that of the main background, meaning that the fringes are not obvious though they still occupy the space given to them by fringe-mode.

Option for line highlighting (hl-line-mode)
===========================================

Symbol names:

  * modus-operandi-theme-intense-hl-line

  * modus-vivendi-theme-intense-hl-line

Possible values:

 1. nil (default)

 2. t

Draw the current line of hl-line-mode or its global equivalent in a more prominent background colour.  This would also affect several packages that enable hl-line-mode, such as elfeed and mu4e.

The default is to use a more subtle grey.

Option for parenthesis matching (show-paren-mode)
=================================================

Symbol names:

  * modus-operandi-theme-intense-paren-match

  * modus-vivendi-theme-intense-paren-match

Possible values:

 1. nil (default)

 2. t

Apply a more intense background to the matching parentheses (or delimiters).  This affects tools such as the built-in show-paren-mode. The default is to use a subtle warm colour for the background of those overlays.

Option for diff buffer looks
============================

Symbol names:

  * modus-operandi-theme-diffs

  * modus-vivendi-theme-diffs

Possible values:

 1. nil (default)

 2. desaturated

 3. fg-only

By default the themes will apply richly coloured backgrounds to the output of diffs, such as those of diff-mode, ediff, smerge-mode, and magit.  These are colour combinations of an accented background and foreground so that, for example, added lines have a pronounced green background with an appropriate shade of green for the affected text. Word-wise or ``refined'' changes follow this pattern but use different shades of those colours to remain distinct.

A desaturated value tones down all relevant colour values.  It still combines an accented background with an appropriate foreground, yet its overall impression is very subtle.  Refined changes are a bit more intense to fulfil their intended function, though still less saturated than default.

While fg-only will remove all accented backgrounds and instead rely on colour-coded text to denote changes.  For instance, added lines use an intense green foreground, while their background is the same as the rest of the buffer.  Word-wise highlights still use a background value which is, nonetheless, more subtle than its default equivalent.

Concerning magit, an extra set of tweaks are introduced for the effect of highlighting the current diff hunk, so as to remain consistent with the overall experience of that mode.  Expect changes that are consistent with the overall intent of the aforementioned.

Option for org-mode block styles
================================

Symbol names:

  * modus-operandi-theme-org-blocks

  * modus-vivendi-theme-org-blocks

Possible values:

 1. nil (default)

 2. greyscale

 3. rainbow

The default is to use the same background as the rest of the buffer for the contents of the block.

A value of greyscale will apply a subtle neutral grey background to the block's contents.  It will also extend to the edge of the window the background of the ``begin'' and ``end'' block delimiter lines (only relevant for Emacs versions >= 27 where the 'extend' keyword is recognised by set-face-attribute).

While rainbow will instead use an accented background for the contents of the block.  The exact colour will depend on the programming language and is controlled by the org-src-block-faces variable (refer to the theme's source code for the current association list).  This is most suitable for users who work on literate programming documents that mix and match several languages.

Note that the ``rainbow'' blocks may require you to also reload the major-mode so that the colours are applied properly: use M-x org-mode or M-x org-mode-restart to refresh the buffer.  Or start typing in each code block (inefficient at scale, but it still works).

Option for headings' overall style
==================================

This is defined as an alist and, therefore, uses a different approach than other customisation options documented in this manual.

Symbol names:

  * modus-operandi-theme-headings

  * modus-vivendi-theme-headings

Possible values, which can be specified for each heading level (examples further below):

  * nil (default fallback option---covers all heading levels)

  * t (default style for a single heading, when the fallback differs)

  * no-bold

  * line

  * line-no-bold

  * rainbow

  * rainbow-line

  * rainbow-line-no-bold

  * highlight

  * highlight-no-bold

  * rainbow-highlight

  * rainbow-highlight-no-bold

  * section

  * section-no-bold

  * rainbow-section

  * rainbow-section-no-bold

To control faces per level from 1-8, use something like this (same for modus-vivendi-theme-headings):

     (setq modus-operandi-theme-headings
           '((1 . section)
             (2 . line)
             (3 . highlight)
             (t . rainbow-no-bold)))

The above uses the section value for heading levels 1, the line for headings 2, highlight for 3.  All other levels fall back to rainbow-line-no-bold.

To set a uniform value for all heading levels, use this pattern:

     ;; A given style for every heading
     (setq modus-operandi-theme-headings
           '((t . rainbow-line-no-bold)))

     ;; Default aesthetic for every heading
     (setq modus-operandi-theme-headings
           '((t . nil)))

The default style for headings uses a fairly desaturated foreground value in combination with a bold typographic weight.  To specify this style for a given level N (assuming you wish to have another fallback option), just specify the value t like this:

     (setq modus-operandi-theme-headings
           '((1 . t)
             (2 . line)
             (t . rainbow-line-no-bold)))

A description of all other possible styles:

  * no-bold retains the default text colour while removing the typographic weight.

  * line is the same as the default plus an overline over the heading.

  * line-no-bold is the same as line without bold weight.

  * rainbow uses a more colourful foreground in combination with bold weight.

  * rainbow-line is the same as rainbow plus an overline.

  * rainbow-line-no-bold is the same as rainbow-line without the bold weight.

  * highlight retains the default style of a fairly desaturated foreground combined with a bold weight and adds to it a subtle accented background.

  * highlight-no-bold is the same as highlight without a bold weight.

  * rainbow-highlight is the same as highlight but with a more colourful foreground.

  * rainbow-highlight-no-bold is the same as rainbow-highlight without a bold weight.

  * section retains the default looks and adds to them both an overline and a slightly accented background.  It is, in effect, a combination of the line and highlight values.

  * section-no-bold is the same as section without a bold weight.

  * rainbow-section is the same as section but with a more colourful foreground.

  * rainbow-section-no-bold is the same as rainbow-section without a bold weight.``

Option for scaled headings
==========================

Symbol names:

  * modus-operandi-theme-scale-headings

  * modus-vivendi-theme-scale-headings

Possible values:

 1. nil (default)

 2. t

Make headings larger in height relative to the main text.  This is noticeable in modes like Org.  The default is to use the same size for headings and body copy.

Control the scale of headings
-----------------------------

In addition to toggles for enabling scaled headings, users can also specify a number of their own.

  * If it is a floating point, say, 1.5, it is interpreted as a multiple of the base font size.  This is the recommended method.

  * If it is an integer, it is read as an absolute font height.  The number is basically the point size multiplied by ten.  So if you want it to be 18pt you must pass 180.  Please understand that setting an absolute value is discouraged, as it will break the layout when you try to change font sizes with the built-in text-scale-adjust command (see Font configurations).

Below are the variables in their default values, using the floating point paradigm.  The numbers are very conservative, but you are free to change them to your liking, such as 1.2, 1.4, 1.6, 1.8, 2.0---or use a resource for finding a consistent scale:

     (setq modus-operandi-theme-scale-1 1.05
           modus-operandi-theme-scale-2 1.1
           modus-operandi-theme-scale-3 1.15
           modus-operandi-theme-scale-4 1.2
           modus-operandi-theme-scale-5 1.3)

     (setq modus-vivendi-theme-scale-1 1.05
           modus-vivendi-theme-scale-2 1.1
           modus-vivendi-theme-scale-3 1.15
           modus-vivendi-theme-scale-4 1.2
           modus-vivendi-theme-scale-5 1.3)

Note that in earlier versions of Org, scaling would only increase the size of the heading, but not of keywords that were added to it, like ``TODO''.  The issue has been fixed upstream: https://protesilaos.com/codelog/2020-09-24-org-headings-adapt/.

Option for variable-pitch font in headings
==========================================

Symbol names:

  * modus-operandi-theme-variable-pitch-headings

  * modus-vivendi-theme-variable-pitch-headings

Possible values:

 1. nil (default)

 2. t

Choose to apply a proportionately spaced, else ``variable-pitch'', typeface to headings (such as in Org mode).  The default is to use the main font family.

Font configurations for Org (and others).

Advanced customisation (do-it-yourself)
***************************************

Unlike the predefined customisation options which follow a straightforward pattern of allowing the user to quickly specify their preference, the themes also provide a more flexible, albeit difficult, mechanism to control things with precision (see Customisation Options).

This section is of interest only to users who are prepared to maintain their own local tweaks and who are willing to deal with any possible incompatibilities between versioned releases of the themes.  As such, they are labelled as ``do-it-yourself'' or ``DIY''.

Full access to the themes' palette
==================================

The variables are:

  * modus-operandi-theme-override-colors-alist

  * modus-vivendi-theme-override-colors-alist

Users can specify an association list that maps the names of colour variables to hexadecimal RGB values (in the form of #RRGGBB).  This means that it is possible to override the entire palette or subsets thereof (see the source code for the actual names and values).

Example:

     ;; Redefine the values of those three variables for the given theme
     (setq modus-vivendi-theme-override-colors-alist
           '(("magenta" . "#ffaabb")
             ("magenta-alt" . "#ee88ff")
             ("magenta-alt-other" . "#bbaaff")))

If you want to be creative, you can define a minor mode that refashions the themes on demand.  The following is a minor mode that gets activated on demand.  We combine it with the function to switch between Modus Operandi and Modus Vivendi (see Toggle between the themes on demand for a basic command, and/or Configure options prior to loading for a more comprehensive setup).

     (define-minor-mode modus-themes-alt-mode
       "Override Modus themes' palette variables with custom values.

     This is intended as a proof-of-concept.  It is, nonetheless, a
     perfectly accessible alternative, conforming with the design
     principles of the Modus themes.  It still is not as good as the
     default colours."
       :init-value nil
       :global t
       (if modus-themes-alt-mode
           (setq modus-operandi-theme-override-colors-alist
                 '(("bg-main" . "#fefcf4")
                   ("bg-dim" . "#faf6ef")
                   ("bg-alt" . "#f7efe5")
                   ("bg-hl-line" . "#f4f0e3")
                   ("bg-active" . "#e8dfd1")
                   ("bg-inactive" . "#f6ece5")
                   ("bg-region" . "#c6bab1")
                   ("bg-header" . "#ede3e0")
                   ("bg-tab-bar" . "#dcd3d3")
                   ("bg-tab-active" . "#fdf6eb")
                   ("bg-tab-inactive" . "#c8bab8")
                   ("fg-unfocused" . "#55556f"))
                 modus-vivendi-theme-override-colors-alist
                 '(("bg-main" . "#100b17")
                   ("bg-dim" . "#161129")
                   ("bg-alt" . "#181732")
                   ("bg-hl-line" . "#191628")
                   ("bg-active" . "#282e46")
                   ("bg-inactive" . "#1a1e39")
                   ("bg-region" . "#393a53")
                   ("bg-header" . "#202037")
                   ("bg-tab-bar" . "#262b41")
                   ("bg-tab-active" . "#120f18")
                   ("bg-tab-inactive" . "#3a3a5a")
                   ("fg-unfocused" . "#9a9aab")))
         (setq modus-operandi-theme-override-colors-alist nil
               modus-vivendi-theme-override-colors-alist nil)))

     (defun modus-themes-toggle (&optional arg)
       "Toggle between `modus-operandi' and `modus-vivendi' themes.

     With optional \\[universal-argument] prefix, enable
     `modus-themes-alt-mode' for the loaded theme."
       (interactive "P")
       (if arg
           (modus-themes-alt-mode 1)
         (modus-themes-alt-mode -1))
       (if (eq (car custom-enabled-themes) 'modus-operandi)
           (progn
             (disable-theme 'modus-operandi)
             (load-theme 'modus-vivendi t))
         (disable-theme 'modus-vivendi)
         (load-theme 'modus-operandi t)))

Font configurations for Org (and others)
========================================

The themes are designed to cope well with mixed font settings (Option for no font mixing).  Currently this applies to org-mode and markdown-mode.

In practice it means that the user can safely opt for a more prose-friendly proportionately spaced typeface as their default, while letting spacing-sensitive elements like tables and inline code always use a monospaced font, by inheriting from the fixed-pitch face.

Users can try the built-in M-x variable-pitch-mode to see the effect in action.

To make everything use your desired font families, you need to configure the variable-pitch (proportional spacing) and fixed-pitch (monospaced) faces respectively.  It may also be convenient to set your main typeface by configuring the default face the same way.

Put something like this in your initialisation file (make sure to read the documentation of set-face-attribute, with M-x describe-function):

     ;; Main typeface
     (set-face-attribute 'default nil :family "DejaVu Sans Mono" :height 110)

     ;; Proportionately spaced typeface
     (set-face-attribute 'variable-pitch nil :family "DejaVu Serif" :height 1.0)

     ;; Monospaced typeface
     (set-face-attribute 'fixed-pitch nil :family "DejaVu Sans Mono" :height 1.0)

Note the differences in the :height property.  The default face must specify an absolute value, which is the point size × 10.  So if you want to use a font at point size 11, you set the height at 110. (:height values do not need to be rounded to multiples of ten: the likes of 115 are perfectly valid—some typefaces will change to account for those finer increments.)  Whereas every other face must have a value that is relative to the default, represented as a floating point (if you use an integer, say, 15 then that means an absolute height).  This is of paramount importantance: it ensures that all fonts can scale gracefully when using something like the text-scale-adjust command which only operates on the base font size (i.e. the default face's absolute height).

An alternative syntax for the default face, is to pass all typeface parameters directly to a font property. (Has the benefit of accepting fontconfig parameters (GNU/Linux), such as "DejaVu Sans Mono-11:hintstyle=hintslight:autohint=false". https://www.freedesktop.org/software/fontconfig/fontconfig-user.html) Note that here we use a standard point size:

     (set-face-attribute 'default nil :font "DejaVu Sans Mono-11")

Again, remember to only ever specify an absolute height for the default.

Org user faces (DIY)
====================

Users of org-mode have the option to configure various keywords and priority cookies to better match their workflow.  User options are org-todo-keyword-faces and org-priority-faces.

As those are meant to be custom faces, it would be futile to have the themes try to guess what each user would want to use, which keywords to target, and so on.  Instead, we can provide guidelines on how to customise things to one's liking with the intent of retaining the overall aesthetics of the theme.

Please bear in mind that the end result of those is not controlled by the active theme but by how Org maps faces to its constructs.  Editing those while org-mode is active requires M-x org-mode-restart for changes to take effect.

Let us assume you wish to visually differentiate your keywords.  You have something like this:

     (setq org-todo-keywords
           '((sequence "TODO(t)" "|" "DONE(D)" "CANCEL(C)")
             (sequence "MEET(m)" "|" "MET(M)")
             (sequence "STUDY(s)" "|" "STUDIED(S)")
             (sequence "WRITE(w)" "|" "WROTE(W)")))

You could then use a variant of the following to inherit from a face that uses the styles you want and also to preserve the properties applied by the org-todo face:

     (setq org-todo-keyword-faces
           '(("MEET" . '(font-lock-preprocessor-face org-todo))
             ("STUDY" . '(font-lock-variable-name-face org-todo))
             ("WRITE" . '(font-lock-type-face org-todo))))

This will refashion the keywords you specify, while letting the other items in org-todo-keywords use their original styles (which are defined in the org-todo and org-done faces).

If you want back the defaults, try specifying just the org-todo face:

     (setq org-todo-keyword-faces
           '(("MEET" . org-todo)
             ("STUDY" . org-todo)
             ("WRITE" . org-todo)))

When you inherit from multiple faces, you need to quote the list as shown further above.  The order is important: the last item is applied over the previous ones.  If you do not want to blend multiple faces, you do not need a quoted list.  A pattern of keyword . face would suffice.

Both approaches can be used simultaneously, as illustrated in this configuration of the priority cookies:

     (setq org-priority-faces
           '((?A . '(org-scheduled-today org-priority))
             (?B . org-priority)
             (?C . '(shadow org-priority))))

To find all the faces that are loaded in your current Emacs session, use M-x list-faces-display.  Also try M-x describe-variable and then specify the name of each of those Org variables demonstrated above.  Their documentation strings will offer you further guidance.

Furthermore, consider reading the ``Notes for aspiring Emacs theme developers'', published on 2020-08-28 by me (Protesilaos Stavrou): https://protesilaos.com/codelog/2020-08-28-notes-emacs-theme-devs/.

Face coverage
*************

Modus Operandi and Modus Vivendi try to provide as close to full face coverage as possible.  This is necessary to ensure a consistently accessible reading experience across all possible interfaces.

Full support for packages or face groups
========================================

This list will always be updated to reflect the current state of the project.  The idea is to offer an overview of the known status of all affected face groups.  The items with an appended asterisk * tend to have lots of extensions, so the ``full support'' may not be 100% true…

  * ace-window

  * ag

  * alert

  * all-the-icons

  * annotate

  * anzu

  * apropos

  * apt-sources-list

  * artbollocks-mode

  * auctex and TeX

  * auto-dim-other-buffers

  * avy

  * awesome-tray

  * binder

  * bm

  * bongo

  * boon

  * breakpoint (provided by the built-in gdb-mi.el library)

  * buffer-expose

  * calendar and diary

  * calfw

  * centaur-tabs

  * change-log and log-view (such as vc-print-log and vc-print-root-log)

  * cider

  * circe

  * color-rg

  * column-enforce-mode

  * company-mode*

  * company-posframe

  * compilation-mode

  * completions

  * counsel*

  * counsel-css

  * counsel-notmuch

  * counsel-org-capture-string

  * cov

  * cperl-mode

  * csv-mode

  * ctrlf

  * custom (M-x customize)

  * dap-mode

  * dashboard (emacs-dashboard)

  * deadgrep

  * debbugs

  * define-word

  * deft

  * dictionary

  * diff-hl

  * diff-mode

  * dim-autoload

  * dir-treeview

  * dired

  * dired-async

  * dired-git

  * dired-git-info

  * dired-narrow

  * dired-subtree

  * diredfl

  * disk-usage

  * doom-modeline

  * dynamic-ruler

  * easy-jekyll

  * easy-kill

  * ebdb

  * ediff

  * eglot

  * el-search

  * eldoc-box

  * elfeed

  * elfeed-score

  * emms

  * enhanced-ruby-mode

  * epa

  * equake

  * erc

  * eros

  * ert

  * eshell

  * eshell-fringe-status

  * eshell-git-prompt

  * eshell-prompt-extras (epe)

  * eshell-syntax-highlighting

  * evil* (evil-mode)

  * evil-goggles

  * evil-visual-mark-mode

  * eww

  * eyebrowse

  * fancy-dabbrev

  * flycheck

  * flycheck-color-mode-line

  * flycheck-indicator

  * flycheck-posframe

  * flymake

  * flyspell

  * flyspell-correct

  * flx

  * freeze-it

  * frog-menu

  * focus

  * fold-this

  * font-lock (generic syntax highlighting)

  * forge

  * fountain (fountain-mode)

  * geiser

  * git-commit

  * git-gutter (and variants)

  * git-lens

  * git-rebase

  * git-timemachine

  * git-walktree

  * gnus

  * golden-ratio-scroll-screen

  * helm*

  * helm-ls-git

  * helm-switch-shell

  * helm-xref

  * helpful

  * highlight-blocks

  * highlight-defined

  * highlight-escape-sequences (hes-mode)

  * highlight-indentation

  * highlight-numbers

  * highlight-symbol

  * highlight-tail

  * highlight-thing

  * hl-defined

  * hl-fill-column

  * hl-line-mode

  * hl-todo

  * hydra

  * hyperlist

  * ibuffer

  * icomplete

  * icomplete-vertical

  * ido-mode

  * iedit

  * iflipb

  * imenu-list

  * indium

  * info

  * info-colors

  * interaction-log

  * ioccur

  * isearch, occur, etc.

  * ivy*

  * ivy-posframe

  * jira (org-jira)

  * journalctl-mode

  * js2-mode

  * julia

  * jupyter

  * kaocha-runner

  * keycast

  * line numbers (display-line-numbers-mode and global variant)

  * lsp-mode

  * lsp-ui

  * magit

  * magit-imerge

  * man

  * markdown-mode

  * markup-faces (adoc-mode)

  * mentor

  * messages

  * minibuffer-line

  * minimap

  * modeline

  * mood-line

  * moody

  * mpdel

  * mu4e

  * mu4e-conversation

  * multiple-cursors

  * neotree

  * no-emoji

  * notmuch

  * num3-mode

  * nxml-mode

  * objed

  * orderless

  * org*

  * org-journal

  * org-noter

  * org-pomodoro

  * org-recur

  * org-roam

  * org-superstar

  * org-table-sticky-header

  * org-treescope

  * origami

  * outline-mode

  * outline-minor-faces

  * package (M-x list-packages)

  * page-break-lines

  * paradox

  * paren-face

  * parrot

  * pass

  * persp-mode

  * perspective

  * phi-grep

  * phi-search

  * pkgbuild-mode

  * pomidor

  * powerline

  * powerline-evil

  * proced

  * prodigy

  * racket-mode

  * rainbow-blocks

  * rainbow-identifiers

  * rainbow-delimiters

  * rcirc

  * regexp-builder (also known as re-builder)

  * rg (rg.el)

  * ripgrep

  * rmail

  * ruler-mode

  * sallet

  * selectrum

  * semantic

  * sesman

  * shell-script-mode

  * show-paren-mode

  * side-notes

  * skewer-mode

  * smart-mode-line

  * smartparens

  * smerge

  * spaceline

  * speedbar

  * spell-fu

  * stripes

  * suggest

  * switch-window

  * swiper

  * swoop

  * sx

  * symbol-overlay

  * syslog-mode

  * table (built-in table.el)

  * telephone-line

  * term

  * tomatinho

  * transient (pop-up windows such as Magit's)

  * trashed

  * treemacs

  * tty-menu

  * tuareg

  * typescript

  * undo-tree

  * vc (built-in mode line status for version control)

  * vc-annotate (C-x v g)

  * vdiff

  * vimish-fold

  * visible-mark

  * visual-regexp

  * volatile-highlights

  * vterm

  * wcheck-mode

  * web-mode

  * wgrep

  * which-function-mode

  * which-key

  * whitespace-mode

  * window-divider-mode

  * winum

  * writegood-mode

  * woman

  * xah-elisp-mode

  * xref

  * xterm-color (and ansi-colors)

  * yaml-mode

  * yasnippet

  * ztree

Plus many other miscellaneous faces that are provided by the upstream GNU Emacs distribution.

Covered indirectly
==================

These do not require any extra styles because they are configured to inherit from some basic faces.  Please confirm.

  * edit-indirect

  * evil-owl

  * perl-mode

  * php-mode

  * rjsx-mode

  * swift-mode

Will NOT be supported
=====================

I have thus far identified a single package that does fit into the overarching objective of this project: solaire (https://github.com/hlissner/emacs-solaire-mode).  It basically tries to cast a less intense background on the main file-visiting buffers, so that secondary elements like sidebars can have the default (pure white/black) background.

I will only cover this package if it ever supports the inverse effect: less intense colours (but still accessible) for supportive interfaces and the intended styles for the content you are actually working on.

Notes for individual packages
*****************************

This section covers information that may be of interest to users of individual packages.

Note on company-mode overlay pop-up
===================================

By default, the company-mode pop-up that lists completion candidates is drawn using an overlay.  This creates alignment issues every time it is placed above a piece of text that has a different height than the default.

The solution recommended by the project's maintainer is to use an alternative front-end for drawing the pop-up which uses child frames instead of overlays. (https://github.com/company-mode/company-mode/issues/1010) (https://github.com/tumashu/company-posframe/)

Note for ERC escaped colour sequences
=====================================

The built-in IRC client erc has the ability to colourise any text using escape sequences that start with ^C (inserted with C-q C-c) and are followed by a number for the foreground and background. (This page explains the basics, though it is not specific to Emacs: https://www.mirc.com/colors.html) Possible numbers are 0-15, with the first entry being the foreground and the second the background, separated by a comma.  Like this ^C1,6.  The minimum setup is this:

     (add-to-list 'erc-modules 'irccontrols)
     (setq erc-interpret-controls-p t
           erc-interpret-mirc-color t)

As this allows users to make arbitrary combinations, it is impossible to guarantee a consistently high contrast ratio.  All we can we do is provide guidance on the combinations that satisfy the accessibility standard of the themes:

Modus Operandi
     Use foreground colour 1 for all backgrounds from 2-15.  Like so: C-q C-c1,N where N is the background.

Modus Vivendi
     Use foreground colour 0 for all backgrounds from 2-13.  Use foreground 1 for backgrounds 14, 15.

Colours 0 and 1 are white and black respectively.  So combine them together, if you must.

Note for powerline or spaceline
===============================

Both Powerline and Spaceline package users will likely need to use the command powerline-reset whenever they make changes to their themes and/or modeline setup.

Note on shr colours
===================

Emacs' HTML rendering mechanism (shr) may need explicit configuration to respect the theme's colours instead of whatever specifications the webpage provides.  Consult C-h v shr-use-colors.

Note for Helm grep
==================

There is one face from the Helm package that is meant to highlight the matches of a grep or grep-like command (ag or ripgrep).  It is helm-grep-match.  However, this face can only apply when the user does not pass --color=always as a command-line option for their command.

Here is the docstring for that face, which is defined in the helm-grep.el library (view a library with M-x find-library).

     Face used to highlight grep matches.  Have no effect when grep backend use ``--color=''

The user must either remove --color from the flags passed to the grep function, or explicitly use --color=never (or equivalent).  Helm provides user-facing customisation options for controlling the grep function's parameters, such as helm-grep-default-command and helm-grep-git-grep-command.

When --color=always is in effect, the grep output will use red text in bold letter forms to present the matching part in the list of candidates.  That style still meets the contrast ratio target of >= 7:1 (accessibility standard WCAG AAA), because it draws the reference to ANSI colour number 1 (red) from the already-supported array of ansi-color-names-vector.

Note on vc-annotate-background-mode
===================================

Due to the unique way vc-annotate (C-x v g) applies colours, support for its background mode (vc-annotate-background-mode) is disabled at the theme level.

Normally, such a drastic measure should not belong in a theme: assuming the user's preferences is bad practice.  However, it has been deemed necessary in the interest of preserving colour contrast accessibility while still supporting a useful built-in tool.

If there actually is a way to avoid such a course of action, without prejudice to the accessibility standard of this project, then please report as much or send patches (see Contributing).

Contributing
************

This section documents the canonical sources of the themes and the ways in which you can contribute to their ongoing development.

Sources of the themes
=====================

The modus-operandi and modus-vivendi themes are built into Emacs. Currently they are in the project's master branch, which is tracking the nominal development release target of 28.0.50.

The source code of the themes is available on Gitlab (https://gitlab.com/protesilaos/modus-themes/), for the time being.  A mirror on Github (https://github.com/protesilaos/modus-themes/) is also on offer.

An HTML version of this manual is available as an extension to the author's personal website (https://protesilaos.com/modus-themes/) (does not rely on any non-free code).

Issues you can help with
========================

A few tasks you can help with:

  * Suggest refinements to packages that are covered.

  * Report packages not covered thus far.

  * Report bugs, inconsistencies, shortcomings.

  * Help expand the documentation of covered-but-not-styled packages.

  * Suggest refinements to the colour palette.

  * Help expand this document or any other piece of documentation.

  * Merge requests for code refinements.

Patches require copyright assignment to the FSF.

It would be great if your feedback also includes some screenshots, GIFs, or short videos, as well as further instructions to reproduce a given setup.  Though this is not a requirement.

Whatever you do, bear in mind the overarching objective of the Modus themes: to keep a contrast ratio that is greater or equal to 7:1 between background and foreground colours.  If a compromise is ever necessary between aesthetics and accessibility, it shall always be made in the interest of the latter.

Patches require copyright assignment to the FSF
===============================================

Code contributions are most welcome.  For any major edit (more than 15 lines, or so, in aggregate per person), you need to make a copyright assignment to the Free Software Foundation.  This is necessary because the themes are part of the upstream Emacs distribution: the FSF must at all times be in a position to enforce the GNU General Public License.

Copyright assignment is a simple process.  Check the request form below (please adapt it accordingly).  You must write an email to the address mentioned in the form and then wait for the FSF to send you a legal agreement.  Sign the document and file it back to them.  This could all happen via email and take about a week.  You are encouraged to go through this process.  You only need to do it once.  It will allow you to make contributions to Emacs in general.

     Please email the following information to assign@gnu.org, and we
     will send you the assignment form for your past and future changes.

     Please use your full legal name (in ASCII characters) as the subject
     line of the message.
     ----------------------------------------------------------------------
     REQUEST: SEND FORM FOR PAST AND FUTURE CHANGES

     [What is the name of the program or package you're contributing to?]

     GNU Emacs

     [Did you copy any files or text written by someone else in these changes?
     Even if that material is free software, we need to know about it.]

     Copied a few snippets from the same files I edited.  Their author,
     Protesilaos Stavrou, has already assigned copyright to the Free Software
     Foundation.

     [Do you have an employer who might have a basis to claim to own
     your changes?  Do you attend a school which might make such a claim?]

     [For the copyright registration, what country are you a citizen of?]

     [What year were you born?]

     [Please write your email address here.]

     [Please write your postal address here.]

     [Which files have you changed so far, and which new files have you written
     so far?]

     Changed a couple of themes that are part of the Emacs source code:

     ./etc/themes/modus-operandi-theme.el
     ./etc/themes/modus-vivendi-theme.el

Acknowledgements
****************

The Modus themes are a collective effort.  Every contribution counts.

Author/maintainer
     Protesilaos Stavrou.

Code contributions
     Anders Johansson, Basil L. Contovounisios, Markus Beppler, Matthew Stevenson.

Ideas and user feedback
     Aaron Jensen, Adam Spiers, Alex Griffin, Alex Peitsinis, Alexey Shmalko, Anders Johansson, André Alexandre Gomes, Arif Rezai, Basil L. Contovounisios, Damien Cassou, Dario Gjorgjevski, David Edmondson, Davor Rotim, Divan Santana, Gerry Agbobada, Gianluca Recchia, Iris Garcia, Len Trigg, Manuel Uberti, Mark Burton, Markus Beppler, Michael Goldenberg, Murilo Pereira, Nicolas De Jaeghere, Pierre Téchoueyres, Roman Rudakov, Ryan Phillips, Shreyas Ragavan, Tassilo Horn, Thibaut Verron, Trey Merkley, Uri Sharf, Utkarsh Singh, Vincent Foley.  As well as users: Ben, Fourchaux, Fredrik, Moesasji, Nick, TheBlob42, dinko, doolio, jixiuf, okamsn, tycho garen.

Packaging
     Dhavan Vaidya (Debian), Stefan Kangas (core Emacs), Stefan Monnier (GNU Elpa).

Inspiration for certain features
     Fabrice Niessen (leuven-theme), Bozhidar Batsov (zenburn-theme).

Meta
****

If you are curious about the principles that govern the development of this project read the essay On the design of the Modus themes (https://protesilaos.com/codelog/2020-03-17-design-modus-themes-emacs/) (2020-03-17).

Here are some more publications for those interested in the kind of work that goes into this project (sometimes the commits also include details of this sort):

  * Modus Operandi theme subtle palette review (https://protesilaos.com/codelog/2020-05-10-modus-operandi-palette-review/) (2020-05-10)

  * Modus Vivendi theme subtle palette review (https://protesilaos.com/codelog/2020-06-13-modus-vivendi-palette-review/) (2020-06-13)

  * Modus themes: new ``faint syntax'' option (https://protesilaos.com/codelog/2020-07-04-modus-themes-faint-colours/) (2020-07-04)

  * Modus themes: major review of ``nuanced'' colours (https://protesilaos.com/codelog/2020-07-08-modus-themes-nuanced-colours/) (2020-07-08)

  * Modus themes: review of blue colours (https://protesilaos.com/codelog/2020-09-14-modus-themes-review-blues/) (2020-09-14)

And here are the canonical sources for this project's documentation:

Manual
     https://protesilaos.com/modus-themes

Change Log
     https://protesilaos.com/modus-themes-changelog

Screenshots
     https://protesilaos.com/modus-themes-pictures

External projects (ports)
*************************

The present section documents projects that extend the scope of the Modus themes.  The following list will be updated whenever relevant information is brought to my attention.  If you already have or intend to produce such a port, feel welcome to contact me (https://protesilaos.com/contact).

Modus exporter
     This is an Elisp library written by Simon Pugnet (https://github.com/polaris64/modus-exporter). Licensed under the terms of the GNU General Public License.  It is meant to capture the colour values of the active Modus theme (Operandi or Vivendi) and output it as a valid theme for some other application.

GNU Free Documentation License
******************************

                     GNU Free Documentation License
                      Version 1.3, 3 November 2008

      Copyright (C) 2000, 2001, 2002, 2007, 2008 Free Software Foundation, Inc.
          <https://fsf.org/>
      Everyone is permitted to copy and distribute verbatim copies
      of this license document, but changing it is not allowed.

     0. PREAMBLE

     The purpose of this License is to make a manual, textbook, or other
     functional and useful document "free" in the sense of freedom: to
     assure everyone the effective freedom to copy and redistribute it,
     with or without modifying it, either commercially or noncommercially.
     Secondarily, this License preserves for the author and publisher a way
     to get credit for their work, while not being considered responsible
     for modifications made by others.

     This License is a kind of "copyleft", which means that derivative
     works of the document must themselves be free in the same sense.  It
     complements the GNU General Public License, which is a copyleft
     license designed for free software.

     We have designed this License in order to use it for manuals for free
     software, because free software needs free documentation: a free
     program should come with manuals providing the same freedoms that the
     software does.  But this License is not limited to software manuals;
     it can be used for any textual work, regardless of subject matter or
     whether it is published as a printed book.  We recommend this License
     principally for works whose purpose is instruction or reference.

     1. APPLICABILITY AND DEFINITIONS

     This License applies to any manual or other work, in any medium, that
     contains a notice placed by the copyright holder saying it can be
     distributed under the terms of this License.  Such a notice grants a
     world-wide, royalty-free license, unlimited in duration, to use that
     work under the conditions stated herein.  The "Document", below,
     refers to any such manual or work.  Any member of the public is a
     licensee, and is addressed as "you".  You accept the license if you
     copy, modify or distribute the work in a way requiring permission
     under copyright law.

     A "Modified Version" of the Document means any work containing the
     Document or a portion of it, either copied verbatim, or with
     modifications and/or translated into another language.

     A "Secondary Section" is a named appendix or a front-matter section of
     the Document that deals exclusively with the relationship of the
     publishers or authors of the Document to the Document's overall
     subject (or to related matters) and contains nothing that could fall
     directly within that overall subject.  (Thus, if the Document is in
     part a textbook of mathematics, a Secondary Section may not explain
     any mathematics.)  The relationship could be a matter of historical
     connection with the subject or with related matters, or of legal,
     commercial, philosophical, ethical or political position regarding
     them.

     The "Invariant Sections" are certain Secondary Sections whose titles
     are designated, as being those of Invariant Sections, in the notice
     that says that the Document is released under this License.  If a
     section does not fit the above definition of Secondary then it is not
     allowed to be designated as Invariant.  The Document may contain zero
     Invariant Sections.  If the Document does not identify any Invariant
     Sections then there are none.

     The "Cover Texts" are certain short passages of text that are listed,
     as Front-Cover Texts or Back-Cover Texts, in the notice that says that
     the Document is released under this License.  A Front-Cover Text may
     be at most 5 words, and a Back-Cover Text may be at most 25 words.

     A "Transparent" copy of the Document means a machine-readable copy,
     represented in a format whose specification is available to the
     general public, that is suitable for revising the document
     straightforwardly with generic text editors or (for images composed of
     pixels) generic paint programs or (for drawings) some widely available
     drawing editor, and that is suitable for input to text formatters or
     for automatic translation to a variety of formats suitable for input
     to text formatters.  A copy made in an otherwise Transparent file
     format whose markup, or absence of markup, has been arranged to thwart
     or discourage subsequent modification by readers is not Transparent.
     An image format is not Transparent if used for any substantial amount
     of text.  A copy that is not "Transparent" is called "Opaque".

     Examples of suitable formats for Transparent copies include plain
     ASCII without markup, Texinfo input format, LaTeX input format, SGML
     or XML using a publicly available DTD, and standard-conforming simple
     HTML, PostScript or PDF designed for human modification.  Examples of
     transparent image formats include PNG, XCF and JPG.  Opaque formats
     include proprietary formats that can be read and edited only by
     proprietary word processors, SGML or XML for which the DTD and/or
     processing tools are not generally available, and the
     machine-generated HTML, PostScript or PDF produced by some word
     processors for output purposes only.

     The "Title Page" means, for a printed book, the title page itself,
     plus such following pages as are needed to hold, legibly, the material
     this License requires to appear in the title page.  For works in
     formats which do not have any title page as such, "Title Page" means
     the text near the most prominent appearance of the work's title,
     preceding the beginning of the body of the text.

     The "publisher" means any person or entity that distributes copies of
     the Document to the public.

     A section "Entitled XYZ" means a named subunit of the Document whose
     title either is precisely XYZ or contains XYZ in parentheses following
     text that translates XYZ in another language.  (Here XYZ stands for a
     specific section name mentioned below, such as "Acknowledgements",
     "Dedications", "Endorsements", or "History".)  To "Preserve the Title"
     of such a section when you modify the Document means that it remains a
     section "Entitled XYZ" according to this definition.

     The Document may include Warranty Disclaimers next to the notice which
     states that this License applies to the Document.  These Warranty
     Disclaimers are considered to be included by reference in this
     License, but only as regards disclaiming warranties: any other
     implication that these Warranty Disclaimers may have is void and has
     no effect on the meaning of this License.

     2. VERBATIM COPYING

     You may copy and distribute the Document in any medium, either
     commercially or noncommercially, provided that this License, the
     copyright notices, and the license notice saying this License applies
     to the Document are reproduced in all copies, and that you add no
     other conditions whatsoever to those of this License.  You may not use
     technical measures to obstruct or control the reading or further
     copying of the copies you make or distribute.  However, you may accept
     compensation in exchange for copies.  If you distribute a large enough
     number of copies you must also follow the conditions in section 3.

     You may also lend copies, under the same conditions stated above, and
     you may publicly display copies.

     3. COPYING IN QUANTITY

     If you publish printed copies (or copies in media that commonly have
     printed covers) of the Document, numbering more than 100, and the
     Document's license notice requires Cover Texts, you must enclose the
     copies in covers that carry, clearly and legibly, all these Cover
     Texts: Front-Cover Texts on the front cover, and Back-Cover Texts on
     the back cover.  Both covers must also clearly and legibly identify
     you as the publisher of these copies.  The front cover must present
     the full title with all words of the title equally prominent and
     visible.  You may add other material on the covers in addition.
     Copying with changes limited to the covers, as long as they preserve
     the title of the Document and satisfy these conditions, can be treated
     as verbatim copying in other respects.

     If the required texts for either cover are too voluminous to fit
     legibly, you should put the first ones listed (as many as fit
     reasonably) on the actual cover, and continue the rest onto adjacent
     pages.

     If you publish or distribute Opaque copies of the Document numbering
     more than 100, you must either include a machine-readable Transparent
     copy along with each Opaque copy, or state in or with each Opaque copy
     a computer-network location from which the general network-using
     public has access to download using public-standard network protocols
     a complete Transparent copy of the Document, free of added material.
     If you use the latter option, you must take reasonably prudent steps,
     when you begin distribution of Opaque copies in quantity, to ensure
     that this Transparent copy will remain thus accessible at the stated
     location until at least one year after the last time you distribute an
     Opaque copy (directly or through your agents or retailers) of that
     edition to the public.

     It is requested, but not required, that you contact the authors of the
     Document well before redistributing any large number of copies, to
     give them a chance to provide you with an updated version of the
     Document.

     4. MODIFICATIONS

     You may copy and distribute a Modified Version of the Document under
     the conditions of sections 2 and 3 above, provided that you release
     the Modified Version under precisely this License, with the Modified
     Version filling the role of the Document, thus licensing distribution
     and modification of the Modified Version to whoever possesses a copy
     of it.  In addition, you must do these things in the Modified Version:

     A. Use in the Title Page (and on the covers, if any) a title distinct
        from that of the Document, and from those of previous versions
        (which should, if there were any, be listed in the History section
        of the Document).  You may use the same title as a previous version
        if the original publisher of that version gives permission.
     B. List on the Title Page, as authors, one or more persons or entities
        responsible for authorship of the modifications in the Modified
        Version, together with at least five of the principal authors of the
        Document (all of its principal authors, if it has fewer than five),
        unless they release you from this requirement.
     C. State on the Title page the name of the publisher of the
        Modified Version, as the publisher.
     D. Preserve all the copyright notices of the Document.
     E. Add an appropriate copyright notice for your modifications
        adjacent to the other copyright notices.
     F. Include, immediately after the copyright notices, a license notice
        giving the public permission to use the Modified Version under the
        terms of this License, in the form shown in the Addendum below.
     G. Preserve in that license notice the full lists of Invariant Sections
        and required Cover Texts given in the Document's license notice.
     H. Include an unaltered copy of this License.
     I. Preserve the section Entitled "History", Preserve its Title, and add
        to it an item stating at least the title, year, new authors, and
        publisher of the Modified Version as given on the Title Page.  If
        there is no section Entitled "History" in the Document, create one
        stating the title, year, authors, and publisher of the Document as
        given on its Title Page, then add an item describing the Modified
        Version as stated in the previous sentence.
     J. Preserve the network location, if any, given in the Document for
        public access to a Transparent copy of the Document, and likewise
        the network locations given in the Document for previous versions
        it was based on.  These may be placed in the "History" section.
        You may omit a network location for a work that was published at
        least four years before the Document itself, or if the original
        publisher of the version it refers to gives permission.
     K. For any section Entitled "Acknowledgements" or "Dedications",
        Preserve the Title of the section, and preserve in the section all
        the substance and tone of each of the contributor acknowledgements
        and/or dedications given therein.
     L. Preserve all the Invariant Sections of the Document,
        unaltered in their text and in their titles.  Section numbers
        or the equivalent are not considered part of the section titles.
     M. Delete any section Entitled "Endorsements".  Such a section
        may not be included in the Modified Version.
     N. Do not retitle any existing section to be Entitled "Endorsements"
        or to conflict in title with any Invariant Section.
     O. Preserve any Warranty Disclaimers.

     If the Modified Version includes new front-matter sections or
     appendices that qualify as Secondary Sections and contain no material
     copied from the Document, you may at your option designate some or all
     of these sections as invariant.  To do this, add their titles to the
     list of Invariant Sections in the Modified Version's license notice.
     These titles must be distinct from any other section titles.

     You may add a section Entitled "Endorsements", provided it contains
     nothing but endorsements of your Modified Version by various
     parties--for example, statements of peer review or that the text has
     been approved by an organization as the authoritative definition of a
     standard.

     You may add a passage of up to five words as a Front-Cover Text, and a
     passage of up to 25 words as a Back-Cover Text, to the end of the list
     of Cover Texts in the Modified Version.  Only one passage of
     Front-Cover Text and one of Back-Cover Text may be added by (or
     through arrangements made by) any one entity.  If the Document already
     includes a cover text for the same cover, previously added by you or
     by arrangement made by the same entity you are acting on behalf of,
     you may not add another; but you may replace the old one, on explicit
     permission from the previous publisher that added the old one.

     The author(s) and publisher(s) of the Document do not by this License
     give permission to use their names for publicity for or to assert or
     imply endorsement of any Modified Version.

     5. COMBINING DOCUMENTS

     You may combine the Document with other documents released under this
     License, under the terms defined in section 4 above for modified
     versions, provided that you include in the combination all of the
     Invariant Sections of all of the original documents, unmodified, and
     list them all as Invariant Sections of your combined work in its
     license notice, and that you preserve all their Warranty Disclaimers.

     The combined work need only contain one copy of this License, and
     multiple identical Invariant Sections may be replaced with a single
     copy.  If there are multiple Invariant Sections with the same name but
     different contents, make the title of each such section unique by
     adding at the end of it, in parentheses, the name of the original
     author or publisher of that section if known, or else a unique number.
     Make the same adjustment to the section titles in the list of
     Invariant Sections in the license notice of the combined work.

     In the combination, you must combine any sections Entitled "History"
     in the various original documents, forming one section Entitled
     "History"; likewise combine any sections Entitled "Acknowledgements",
     and any sections Entitled "Dedications".  You must delete all sections
     Entitled "Endorsements".

     6. COLLECTIONS OF DOCUMENTS

     You may make a collection consisting of the Document and other
     documents released under this License, and replace the individual
     copies of this License in the various documents with a single copy
     that is included in the collection, provided that you follow the rules
     of this License for verbatim copying of each of the documents in all
     other respects.

     You may extract a single document from such a collection, and
     distribute it individually under this License, provided you insert a
     copy of this License into the extracted document, and follow this
     License in all other respects regarding verbatim copying of that
     document.

     7. AGGREGATION WITH INDEPENDENT WORKS

     A compilation of the Document or its derivatives with other separate
     and independent documents or works, in or on a volume of a storage or
     distribution medium, is called an "aggregate" if the copyright
     resulting from the compilation is not used to limit the legal rights
     of the compilation's users beyond what the individual works permit.
     When the Document is included in an aggregate, this License does not
     apply to the other works in the aggregate which are not themselves
     derivative works of the Document.

     If the Cover Text requirement of section 3 is applicable to these
     copies of the Document, then if the Document is less than one half of
     the entire aggregate, the Document's Cover Texts may be placed on
     covers that bracket the Document within the aggregate, or the
     electronic equivalent of covers if the Document is in electronic form.
     Otherwise they must appear on printed covers that bracket the whole
     aggregate.

     8. TRANSLATION

     Translation is considered a kind of modification, so you may
     distribute translations of the Document under the terms of section 4.
     Replacing Invariant Sections with translations requires special
     permission from their copyright holders, but you may include
     translations of some or all Invariant Sections in addition to the
     original versions of these Invariant Sections.  You may include a
     translation of this License, and all the license notices in the
     Document, and any Warranty Disclaimers, provided that you also include
     the original English version of this License and the original versions
     of those notices and disclaimers.  In case of a disagreement between
     the translation and the original version of this License or a notice
     or disclaimer, the original version will prevail.

     If a section in the Document is Entitled "Acknowledgements",
     "Dedications", or "History", the requirement (section 4) to Preserve
     its Title (section 1) will typically require changing the actual
     title.

     9. TERMINATION

     You may not copy, modify, sublicense, or distribute the Document
     except as expressly provided under this License.  Any attempt
     otherwise to copy, modify, sublicense, or distribute it is void, and
     will automatically terminate your rights under this License.

     However, if you cease all violation of this License, then your license
     from a particular copyright holder is reinstated (a) provisionally,
     unless and until the copyright holder explicitly and finally
     terminates your license, and (b) permanently, if the copyright holder
     fails to notify you of the violation by some reasonable means prior to
     60 days after the cessation.

     Moreover, your license from a particular copyright holder is
     reinstated permanently if the copyright holder notifies you of the
     violation by some reasonable means, this is the first time you have
     received notice of violation of this License (for any work) from that
     copyright holder, and you cure the violation prior to 30 days after
     your receipt of the notice.

     Termination of your rights under this section does not terminate the
     licenses of parties who have received copies or rights from you under
     this License.  If your rights have been terminated and not permanently
     reinstated, receipt of a copy of some or all of the same material does
     not give you any rights to use it.

     10. FUTURE REVISIONS OF THIS LICENSE

     The Free Software Foundation may publish new, revised versions of the
     GNU Free Documentation License from time to time.  Such new versions
     will be similar in spirit to the present version, but may differ in
     detail to address new problems or concerns.  See
     https://www.gnu.org/licenses/.

     Each version of the License is given a distinguishing version number.
     If the Document specifies that a particular numbered version of this
     License "or any later version" applies to it, you have the option of
     following the terms and conditions either of that specified version or
     of any later version that has been published (not as a draft) by the
     Free Software Foundation.  If the Document does not specify a version
     number of this License, you may choose any version ever published (not
     as a draft) by the Free Software Foundation.  If the Document
     specifies that a proxy can decide which future versions of this
     License can be used, that proxy's public statement of acceptance of a
     version permanently authorizes you to choose that version for the
     Document.

     11. RELICENSING

     "Massive Multiauthor Collaboration Site" (or "MMC Site") means any
     World Wide Web server that publishes copyrightable works and also
     provides prominent facilities for anybody to edit those works.  A
     public wiki that anybody can edit is an example of such a server.  A
     "Massive Multiauthor Collaboration" (or "MMC") contained in the site
     means any set of copyrightable works thus published on the MMC site.

     "CC-BY-SA" means the Creative Commons Attribution-Share Alike 3.0
     license published by Creative Commons Corporation, a not-for-profit
     corporation with a principal place of business in San Francisco,
     California, as well as future copyleft versions of that license
     published by that same organization.

     "Incorporate" means to publish or republish a Document, in whole or in
     part, as part of another Document.

     An MMC is "eligible for relicensing" if it is licensed under this
     License, and if all works that were first published under this License
     somewhere other than this MMC, and subsequently incorporated in whole or
     in part into the MMC, (1) had no cover texts or invariant sections, and
     (2) were thus incorporated prior to November 1, 2008.

     The operator of an MMC Site may republish an MMC contained in the site
     under CC-BY-SA on the same site at any time before August 1, 2009,
     provided the MMC is eligible for relicensing.

     ADDENDUM: How to use this License for your documents

     To use this License in a document you have written, include a copy of
     the License in the document and put the following copyright and
     license notices just after the title page:

         Copyright (c)  YEAR  YOUR NAME.
         Permission is granted to copy, distribute and/or modify this document
         under the terms of the GNU Free Documentation License, Version 1.3
         or any later version published by the Free Software Foundation;
         with no Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts.
         A copy of the license is included in the section entitled "GNU
         Free Documentation License".

     If you have Invariant Sections, Front-Cover Texts and Back-Cover Texts,
     replace the "with...Texts." line with this:

         with the Invariant Sections being LIST THEIR TITLES, with the
         Front-Cover Texts being LIST, and with the Back-Cover Texts being LIST.

     If you have Invariant Sections without Cover Texts, or some other
     combination of the three, merge those two alternatives to suit the
     situation.

     If your document contains nontrivial examples of program code, we
     recommend releasing these examples in parallel under your choice of
     free software license, such as the GNU General Public License,
     to permit their use in free software.