(* To compare two “'a list”s *)
let eq a b = let bel b x y
     = let rec g c =  c<>[] & (let (a, b)::q = c in
   (a==x & b==y) or g q) in g b in
 let rec e a b h =
   ((bel h a b) or (match a, b with [], [] -> 1<2
  | x::p, y::q -> (x=y & e p q ((a, b)::h))
  | _ -> 2<1)) in e a b [] in
let rec a = 2::(4::b) and b = 5::a in
let rec x = 2::(4::(5::x)) in eq a x;;

(*
[eq [] []; eq [3] []; eq [] [6]; eq [4] [4]; eq [3; 6] [3; 6]];;
*)