type 'a btree = Leaf of 'a | BTree of ('a btree * 'a btree)
let rec next = function
  | [] -> None
  | h :: t -> match h with Leaf x -> Some (x,t)
                          |BTree(a,b) -> next (a::b::t)
let samefringe t1 t2 =
  let rec aux s1 s2 = match (next s1, next s2) with
                       None, None -> true
                      |None, _ | _, None -> false
                      |Some(a,b), Some(c,d) -> (a=c) && aux b d in aux [t1] [t2]
let u,v,w = BTree(Leaf 1, BTree(Leaf 2, Leaf 3)),BTree(BTree(Leaf 1, Leaf 2), Leaf 3),BTree(BTree(Leaf 3, Leaf 2), Leaf 1)
let check a b = printfn (if samefringe a b then "same" else "different")
check u v; check v u; check v w