(* To compare two “'a tree”s *)
type 'a tree = NodeTree of ('a tree * 'a tree) | LeafTree of 'a;;
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 h a b =
   ((bel h a b) or (match a, b with
    LeafTree x, LeafTree y -> x=y
  | NodeTree (g, f), NodeTree (c, d) ->
       let p = e ((a, b)::h) in (p g c & p f d)
  | _ -> 2<1)) in e [] a b in
let rec a = NodeTree (NodeTree(a, LeafTree 3), a) in
let rec b = NodeTree (b, NodeTree(LeafTree 3, b)) in
  print_int (if eq (NodeTree(LeafTree 3, a)) b then 1 else 0);;