Infix to postfix expression conversion

infix-postfix.lisp

(defun precedence (op)
  (cond
    ((or (eq op '*) (eq op '/)) 20)
    ((or (eq op '+) (eq op '-)) 10)
    (t 0)))

(defun infix->postfix (infix-exp)
  "infix->postfix returns a postfix version of the infix expression passed to it. Currently, it does no error checking and does not support expressions with brackets."
  (labels ((helper (acc op el iexp)
             (cond
               ((numberp el) (helper (cons el acc) op (car iexp) (cdr iexp)))
               ((null iexp) (nconc (nreverse acc) op))
               ((or (null op) (> (precedence el) (precedence (car op))))
                (helper acc (cons el op) (car iexp) (cdr iexp)))
               (t (helper (cons (car op) acc) (cdr op) el iexp)))))
    (helper () () (car infix-exp) (cdr infix-exp))))