[jff]

You may be right, maybe I've just only looked at Scheme lately and thought "?" had migrated to Common Lisp.

https://www.cs.cmu.edu/Groups/AI/html/cltl/clm/node188.html seems to indicate that you could use ? in a common lisp function name, though.

[throwanem]

You could, but I've never seen it. Convention seems to be to suffix 'p' if the predicate name is a single word, and '-p' otherwise.