count-atoms.lisp

(defun count-atoms (exp)
  "Return the total number of non-nil atoms in the expression"
  (cond ((null exp) 0)
	((atom exp) 1)
	(t (+ (count-atoms (car exp))
	      (count-atoms (cdr exp))))))


(defun count-all-atoms (exp &optional (if-null 1))
  "Return the total number of atoms in the expression,
   counting nil as an atom only in non-tail position"
  (cond ((null exp) if-null)
	((atom exp 1)
	 (t (+ (count-all-atoms (first exp) 1)
	    (count-all-atoms (rest exp) 0))))))