TRS:
 { double(0()) -> 0(),
  double(s(x)) -> s(s(double(x))),
     +(x, 0()) -> x,
    +(x, s(y)) -> s(+(x, y)),
    +(s(x), y) -> s(+(x, y)),
     double(x) -> +(x, x)}
 MPO:
  Prec:
   double > +, 
   double > s, 
   + > s
   empty
  Strict:
   {}
   Weak:
    {}
  Qed