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

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

Is there a check, e.g. optional, for whether a given character cdr
for a `prettify-symbols-alist' entry is, as the only char in a
symbol name, either `fboundp' or `boundp', before going ahead and
prettifying the symbol that is the cdr of that alist entry?

IOW, check whether, e.g., for an entry such as `("not" . ?¬)',
(boundp '¬) or (fboundp '¬)?

A cursory glance at the code didn't find such a check.  Seems like
it might be a good thing to add.  If `¬' already exists as a
function or variable, then you probably do not also want
occurrences of the symbol `not' to be shown as `¬'.

IOW, if the only guard for prettifying is `prettify-symbols-alist'
then there could be cases of symbol-appearance capture.  Sure,
this would not be common, and sure, someone could turn off the
mode if it is noticed.  But it might also be good to make such
a check available (optional).