RE: ¬ notation for not? Ref: Add a couple cells to lisp-prettify-symbols-alist

[Drew Adams <drew.adams@oracle.com>]

> SM> Hopefully someone will work on adding support for user-customization so
> SM> we can have other options than "include it for all, or include it for
> SM> none".
> 
> I proposed tagging each prettification entry, so the user can customize
> the tags. You said it was not right; what kind of customization are you
> proposing? I hope we agree on it here before it gets committed.

And I proposed guarding prettification not only by checking
`prettify-symbols-alist' for a match, by also by ensuring
that the function or variable of a match is not `fboundp'
or `boundp'.

At least do so optionally (i.e., customization).

E.g., ensure that "¬" does not name a symbol that has a
function or variable value.  If `¬' is already bound to some
function or variable then you probably do not want to alias
function `not' to it.

Emacs allows most characters in function and variable names.
Just because `prettify-symbols-alist' might have an entry for
a given string, that does not mean that the user wants to
clobber any existing function or variable that has that name.