Recognizer for a ``message''
The form (msgp x) evaluates to true when x evaluates either to a string or to a cons whose car is a string and whose cdr satisfies character-alistp. Note that msgp will always hold for the output of the macro msg.
Function: msgp
(defun msgp (x) (declare (xargs :guard t)) (or (stringp x) (and (consp x) (stringp (car x)) (character-alistp (cdr x)))))
