[maksimum]

As a python programmer I would have no idea what `valid_P` is supposed to mean. Valid Pascals?

[bandrami]

P is shaped like a question mark, though I think it was retronymed to stand for "Predicate"

[NateEag]

I think "predicate" is the original meaning of that postfix.

Emacs Lisp, at least, can use question marks in names without batting an eyelash.

I see no reason that would not have been true for Lisps generally since their birth.