(*
   Using FIRST_ARG and FILTER write a function THRESHOLD that takes an integer X
   and returns a function that takes a list L and returns the elements of L
   that are less than or equal to X.
      e.g. (THRESHOLD 3) [1,2,3,4,5,6]  should return [1,2,3]
*)

use "FILTER";

local
    fun KEEP (A:int,B) = not (B > A)  (* true if B doesn't exceed threshold A *)
in
fun THRESHOLD X = FIRST_ARG (FILTER, FIRST_ARG (KEEP,X))
end;