Tests is an object is a character or character repertoire.
common-lisp
characterp object &optional repertoire => bool
object
The object to be tested.
repertoire
A character repertoire.
bool
t if object is a character (or character repertoire, if repertoire is given); nil otherwise.
t
nil
This is the predicate for characters (when repertoire is not given), or character repertoires.
character
CL-USER 234 > (characterp #\a :ascii)
T