MAYBE

Problem:
 isLeaf(leaf()) -> true()
 isLeaf(cons(u,v)) -> false()
 left(cons(u,v)) -> u
 right(cons(u,v)) -> v
 concat(leaf(),y) -> y
 concat(cons(u,v),y) -> cons(u,concat(v,y))
 less_leaves(u,v) -> if1(isLeaf(u),isLeaf(v),u,v)
 if1(b,true(),u,v) -> false()
 if1(b,false(),u,v) -> if2(b,u,v)
 if2(true(),u,v) -> true()
 if2(false(),u,v) -> less_leaves(concat(left(u),right(u)),concat(left(v),right(v)))

Proof:
 Open