(use '[clojure.core.match :only [match]])

(defn evaluate [env exp]
  (match [exp]
    [(['Number a] :seq)] a
    [(['Add x y] :seq)] (+ (evaluate env x) (evaluate env y))
    [(['Multiply x y] :seq)] (* (evaluate env x) (evaluate env y))
    [(['Variable i] :seq)] (env i)))

(def environment {"a" 3, "b" 4, "c" 5})

(def expression-tree '(Add (Variable "a") (Multiply (Number 2) (Variable "b"))))

(def result (evaluate environment expression-tree))