[U,V,W,X,Y,Z]

concat(leaf,Y) ->  Y.
concat(cons(U,V),Y) -> cons(U,concat(V,Y)).
lessleaves(X,leaf) -> false.
lessleaves(leaf,cons(W,Z)) -> true.
lessleaves(cons(U,V),cons(W,Z)) -> lessleaves(concat(U,V),concat(W,Z)).

