(defn diff [expr var]
  (cond
    (symbol? expr) (if (= expr var) 1 0)
    (number? expr) 0
    (match expr
      [op & args] (case op
        '+ ['+ ;(map |(diff $ var) args)]
        '* (let [[a b] args]
             ['+ ['* (diff a var) b]
                        ['* a (diff b var)]])
        (errorf "unknown op: %v" op)))))

(pp (diff '(+ (* x x) 3) 'x))
(pp (diff '(+ x 3) 'x))