visual distinction for prettified symbols (was: ¬ notation for not? Ref: Add a couple cells to lisp-prettify-symbols-alist)

[Ted Zlatanov <tzz@lifelogs.com>]

On Fri, 15 Jul 2016 07:46:56 -0700 (PDT) Drew Adams <drew.adams@oracle.com> wrote: 

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

That's not really related to my question about customizations, and there
is no aliasing going on in any case. The user will see ¬ in both cases,
but they will be different underneath.

`prettify-symbols-unprettify-at-point' set to 'right-edge is really
helpful in this case, incidentally.

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

It's a visual clobbering, but yeah, I know what you mean. Typically
users will not have such functions or variables, but it would be good to
distinguish them in a way that makes their ephemeral nature clear. Right
now they aren't.

The "shadow" face looks like a great candidate. Or maybe there can be a
new "prettified" face, similar to "shadow"? It should have colors and
decorations that are not used much otherwise, so "dimmed out" like the
"shadow" face might be ideal.

Ted