(type B (enum (B (x i32) (y i32))))

;; `isub` has a constructor and extractor.
(decl isub (i32 i32) B)
(rule (isub x y)
      (B.B x y))
(extractor (isub x y)
           (B.B x y))

;; `value_array_2` has both an external extractor and an external constructor.
(type Value (primitive Value))
(type ValueArray2 extern (enum))
(decl value_array_2 (Value Value) ValueArray2)
(extern extractor infallible value_array_2 unpack_value_array_2)
(extern constructor value_array_2 pack_value_array_2)