(= zero-t (data))
(= succ-t (data prev))

(= zero (zero-c))
(= one (succ-c zero))
(= two (succ-c one))
(= three (succ-c two))
(= four (succ-c three))

;; method

(= (succ-t.inc self)
   (succ-c self))

(assert (eq (succ-t.inc one) two))
(assert (eq (succ-t.inc two) three))

;; associated function

(= (null-t.cons x)
   (cons x null))

(assert (eq (null-t.cons null)
            (cons null null)))