open SMLofNJ.Cont (* open module that provides continuations *)

datatype tree = leaf of int | node of tree*tree

datatype coA = A of (int* coB) cont   (* searchA wants an int and a B-continuation *)
and      coB = B of       coA  cont  (* searchB wants an A-continuation *)

fun resumeA(x, A k) = callcc(fn k' => throw k (x, B k'))
fun resumeB(   B k) = callcc(fn k' => throw k (A k'))

exception DISAGREE
exception DONE

fun searchA(leaf(x),(y, other: coB)) = 
         if x=y then resumeB(other) else raise DISAGREE
|   searchA(node(t1,t2), other) = searchA(t2, searchA(t1, other))

fun searchB(leaf(x), other : coA) = resumeA(x,other) 
|   searchB(node(t1,t2), other) = searchB(t2, searchB(t1, other))

fun startB(t: tree)  =  callcc(fn k => (searchB(t, A k); raise DONE)) 

fun compare(t1,t2) = searchA(t1, startB(t2))