MAYBE

Problem:
 f(true(),x,y) -> f(gt(x,y),double(x),s(y))
 gt(s(x),0()) -> true()
 gt(0(),y) -> false()
 gt(s(x),s(y)) -> gt(x,y)
 double(x) -> times(x,s(s(0())))
 times(0(),y) -> 0()
 times(s(x),y) -> plus(y,times(x,y))
 plus(0(),y) -> y
 plus(s(x),y) -> plus(x,s(y))

Proof:
 Open