Lisp macro wizardry

gistfile1.lisp

(defmacro diff(expr var)
 ; (pprint "diff")
 ; (pprint  expr)
 ; (pprint var)
 ;(pprint (first expr))
 ;(pprint (second expr))
 ;(pprint (third expr))
 (cond
  ; a' = 0
  ((numberp expr) 0)

  ; x' = 1, y' = 0
  ((symbolp expr)
   (if (eq expr var) 1 0))

  ; sum
  ((equal (first expr) '+) `(list '+ (diff ,(second expr) ,var) (diff ,(third expr) ,var)))
 ))

(diff (+ a a) c)

(macroexpand '(diff (+ a a) c))