YES(?,O(n^2))
TRS:
 {
                merge(x, nil()) -> x,
                merge(nil(), y) -> y,
  merge(++(x, y), ++(u(), v())) -> ++(x, merge(y, ++(u(), v()))),
  merge(++(x, y), ++(u(), v())) -> ++(u(), merge(++(x, y), v()))
 }
 DUP: We consider a non-duplicating system.
  Trs:
   {
                  merge(x, nil()) -> x,
                  merge(nil(), y) -> y,
    merge(++(x, y), ++(u(), v())) -> ++(x, merge(y, ++(u(), v()))),
    merge(++(x, y), ++(u(), v())) -> ++(u(), merge(++(x, y), v()))
   }
  Matrix Interpretation:
   Interpretation class: triangular
         [1]
   [v] = [1]
   
        [X3]  [X1]    [1 2][X3]   [1 0][X1]   [1]
   [++]([X2], [X0]) = [0 0][X2] + [0 1][X0] + [2]
   
           [X3]  [X1]    [1 3][X3]   [1 2][X1]   [2]
   [merge]([X2], [X0]) = [0 1][X2] + [0 1][X0] + [0]
   
           [1]
   [nil] = [0]
   
         [1]
   [u] = [3]
   
   
   Qed