($ a [1 true 2])
(assert-eq? 1 (ref a 0))
(assert-eq? true (ref a 1))
(assert-eq? 2 (ref a 2))

($ b [[2 4 6] [8 10 12] [14 16 18]])
(assert-eq? 12 (ref b 1 2))

(assert-eq? true (empty? []))
(assert-eq? false (empty? [1]))

(assert-eq? 7 (head [7 3 1]))
(assert-eq? 3 (ref (tail [7 3 1]) 0))
(assert-eq? 1 (ref (tail [7 3 1]) 1))

($ (c arr)
  (if (empty? arr)
    0
    (+ (head arr) (c (tail arr)))))

(assert-eq? 13 (c [2 4 6 1]))

($ d (cons 2 (cons 4 [6])))

(assert-eq? 2 (ref d 0))
(assert-eq? 4 (ref d 1))
(assert-eq? 6 (ref d 2))

(assert-eq? 3 (len d))
(assert-eq? 4 (len (cons 18 d)))