maths-list-p

; A maths list is of the form:
; (r d d d ...)
; where r is an operator
; and d is either a maths list or a number.

(defun maths-list-p (expr)
  (if (not (member (first expr) '(+ - * /))) ;check r
      NIL
      (loop
       for i in (rest expr)
       if (not (or (numberp i) (maths-list-p i))) return NIL ;check d
       finally return T)))