MAYBE
Trs:
 {                  g(C()) -> C(),
                    g(C()) -> B(),
                    g(C()) -> A(),
                    g(B()) -> B(),
                    g(B()) -> A(),
                    g(A()) -> A(),
  f'(triple(a, b, c), C()) -> triple(a, b, s(c)),
  f'(triple(a, b, c), B()) -> f(triple(a, b, c), A()),
  f'(triple(a, b, c), A()) -> f''(foldB(triple(s(a), 0(), c), b)),
             foldB(t, 0()) -> t,
            foldB(t, s(n)) -> f(foldB(t, n), B()),
      f''(triple(a, b, c)) -> foldC(triple(a, b, 0()), c),
             foldC(t, 0()) -> t,
            foldC(t, s(n)) -> f(foldC(t, n), C()),
           fold(t, x, 0()) -> t,
          fold(t, x, s(n)) -> f(fold(t, x, n), x),
                   f(t, x) -> f'(t, g(x))}
 Comment:
  We consider a duplicating trs.
  FAIL:
   Open