MAYBE

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

Proof:
 Open