(defn rator [e] (first e))
(defn rands [e] (rest e))

(defn cst? [n] (number? n))
(defn sym? [v] (string? v))
(defn op? [e] (vector? e))
(defn add? [e] (and (op? e) (= (rator e) :add)))
(defn mul? [e] (and (op? e) (= (rator e) :mul)))