MAYBE

Problem:
 f(true(),true(),x,s(y)) -> f(isNat(x),isNat(y),s(x),double(s(y)))
 isNat(0()) -> true()
 isNat(s(x)) -> isNat(x)
 double(0()) -> 0()
 double(s(x)) -> s(s(double(x)))

Proof:
 Open