(*
Filtering, with a predicate, based on all possible comparisions by pair, of the
elements from an input list:
*)

(** filter, with a predicate, from all combinations of 2 elements from n *)

let ls = [1; 2; 3; 4; 5; 6]

let comb2_filter p ls =
  let rec aux ls acc =
    match ls with
    | [] -> (List.rev acc)
    | x::xs ->
        let _acc, ys =
          List.fold_left (fun (acc, ys) y ->
            if (p x y)
            then (acc, y::ys)
            else (false, ys)
          ) (true, []) xs
        in
        let acc = if _acc then x::acc else acc in
        let ys = List.rev ys in
        aux ys acc
  in
  aux ls []

let () =
  let fs =
    comb2_filter (fun v1 v2 ->
      match (v1, v2) with
      | (5, 4) | (4, 5) -> false
      | _ -> true
    ) ls
  in
  List.iter (fun (v) ->
    Printf.printf " (%d)\n" v
  ) fs

(*
Result:
*)

(*
 (1)
 (2)
 (3)
 (6)
*)