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