MAYBE

Problem:
 sub(0(),0()) -> 0()
 sub(s(x),0()) -> s(x)
 sub(0(),s(x)) -> 0()
 sub(s(x),s(y)) -> sub(x,y)
 zero(nil()) -> zero2(0(),nil())
 zero(cons(x,xs)) -> zero2(sub(x,x),cons(x,xs))
 zero2(0(),nil()) -> nil()
 zero2(0(),cons(x,xs)) -> cons(sub(x,x),zero(xs))
 zero2(s(y),nil()) -> zero(nil())
 zero2(s(y),cons(x,xs)) -> zero(cons(x,xs))

Proof:
 Open