YES(?,O(n^1))
TRS:
 {f s s x -> f f s x,
  f s 0() -> s 0(),
    f 0() -> s 0()}
 DUP: We consider a non-duplicating system.
  Trs:
   {f s s x -> f f s x,
    f s 0() -> s 0(),
      f 0() -> s 0()}
  BOUND:
   Bound: match(-raise)-bounded by 3
   Automaton:
    {
        0_3() -> 14*
        0_2() -> 12*
        0_1() -> 2*
        0_0() -> 1*
      s_3(14) -> 15*
      s_2(12) -> 13*
       s_2(8) -> 9*
      s_1(16) -> 17*
       s_1(4) -> 5*
       s_1(2) -> 3*
       s_0(1) -> 1*
      f_2(18) -> 19*
      f_2(10) -> 11*
       f_2(9) -> 10*
       f_1(6) -> 7*
       f_1(5) -> 6*
     f_0(1) -> 1*
         19 -> 6*
         17 -> 6*
         15 -> 19 | 11
         13 -> 10 | 7
         12 -> 16*
         11 -> 18 | 6
          7 -> 6 | 1
          3 -> 6 | 1
          2 -> 8*
          1 -> 4*
    }
   Strict:
    {}
   Qed