YES(?,O(n^2))
TRS:
 {
    a__f(X1, X2, X3) -> f(X1, X2, X3),
     a__f(a(), X, X) -> a__f(X, a__b(), b()),
              a__b() -> b(),
              a__b() -> a(),
  mark f(X1, X2, X3) -> a__f(X1, mark X2, X3),
            mark b() -> a__b(),
            mark a() -> a()
 }
 DUP: We consider a non-duplicating system.
  Trs:
   {
      a__f(X1, X2, X3) -> f(X1, X2, X3),
       a__f(a(), X, X) -> a__f(X, a__b(), b()),
                a__b() -> b(),
                a__b() -> a(),
    mark f(X1, X2, X3) -> a__f(X1, mark X2, X3),
              mark b() -> a__b(),
              mark a() -> a()
   }
  Matrix Interpretation:
   Interpretation class: triangular
          [X1]    [1 1][X1]   [2]
   [mark]([X0]) = [0 1][X0] + [1]
   
         [0]
   [a] = [2]
   
         [0]
   [b] = [1]
   
            [1]
   [a__b] = [2]
   
          [X5]  [X3]  [X1]    [1 3][X5]   [1 0][X3]   [1 3][X1]   [2]
   [a__f]([X4], [X2], [X0]) = [0 1][X4] + [0 1][X2] + [0 0][X0] + [3]
   
       [X5]  [X3]  [X1]    [1 3][X5]   [1 0][X3]   [1 3][X1]   [1]
   [f]([X4], [X2], [X0]) = [0 1][X4] + [0 1][X2] + [0 0][X0] + [3]
   
   
   Qed