($ a 2)

(:
  (assert= 2 a)
  ($ a 3)
  (assert= 3 a)

  (:
    ($ a 7)
    (assert= 7 a)
    (assert-static-fail ($ a 2))
   )

   (assert= 3 a)
)

(assert= 2 a)

($ b (: 2 6 9))

(assert= 9 b)

($ c (: 1 2 (: 3 4 (: 5 6 7))))
(assert= 7 c)