MAYBE

Problem:
 -(x,0()) -> x
 -(s(x),s(y)) -> -(x,y)
 +(0(),y) -> y
 +(s(x),y) -> s(+(x,y))
 *(x,0()) -> 0()
 *(x,s(y)) -> +(x,*(x,y))
 f(s(x),y) -> f(-(*(s(x),s(y)),s(*(s(x),y))),*(y,y))

Proof:
 Open