MAYBE

Problem:
 minus(x,y) -> cond(ge(x,s(y)),x,y)
 cond(false(),x,y) -> 0()
 cond(true(),x,y) -> s(minus(x,s(y)))
 ge(u,0()) -> true()
 ge(0(),s(v)) -> false()
 ge(s(u),s(v)) -> ge(u,v)

Proof:
 Open