(module c1_local_hof
  (data Box
    (doc "heap cell")
    (ctor Box (con Int)))
  (data List
    (doc "boxed list of boxes")
    (ctor Nil)
    (ctor Cons (con Box) (con List)))
  (fn filter_box
    (doc "borrow-mode predicate p applied to h; h reused in the kept Cons")
    (type
      (fn-type
        (params (borrow (fn-type (params (borrow (con Box))) (ret (own (con Bool)))))
                (own (con List)))
        (ret (own (con List)))))
    (params p xs)
    (body
      (match xs
        (case (pat-ctor Nil) (term-ctor List Nil))
        (case (pat-ctor Cons h t)
          (if (app p h)
            (term-ctor List Cons h (app filter_box p t))
            (app filter_box p t)))))))