YES
O(n^4)
TRS:
 {f(f(a())) -> c()}
 DUP: We consider a non-duplicating system.
  Trs:
   {f(f(a())) -> c()}
  Matrix Interpretation:
   Interpretation class: triangular
         [0]
         [0]
   [a] = [0]
         [0]
   
       [X3]    [1 0 0 1][X3]   [0]
       [X2]    [0 0 0 1][X2]   [1]
   [f]([X1]) = [0 0 0 0][X1] + [0]
       [X0]    [0 0 0 1][X0]   [1]
   
         [0]
         [0]
   [c] = [0]
         [0]
   
   
   Qed