### example.ml [ocaml]
open Batteries

module type M =
sig
  include Map.S
  val filter: (key -> 'a -> bool) -> 'a t -> 'a t
end

module BetterMap (OM: Map.S): (M with type key = OM.key) =
  struct
    include OM

    let filter pred m =
      fold
        (fun k v accum ->
          if pred k v then
            add k v accum
          else
            accum)
        m
        empty
  end

module IntMap = BetterMap(IntMap)
### error.txt

       Values do not match:
         val filter : (key -> 'a -> bool) -> 'a t -> 'a t
       is not included in
         val filter : ('a -> bool) -> 'a t -> 'a t