MAYBE
Trs:
 {                 sum(nil(), y) -> y,
  sum(cons(s(n), x), cons(m, y)) -> sum(cons(n, x), cons(s(m), y)),
            sum(cons(0(), x), y) -> sum(x, y),
          weight(cons(n, nil())) -> n,
     weight(cons(n, cons(m, x))) -> weight(sum(cons(n, cons(m, x)), cons(0(), x)))}
 Comment:
  We consider a duplicating trs.
  FAIL:
   Open