YES
O(n^2)
TRS:
 {
                    concat(leaf(), y) -> y,
                concat(cons(u, v), y) -> cons(u, concat(v, y)),
               less_leaves(x, leaf()) -> false(),
      less_leaves(leaf(), cons(w, z)) -> true(),
  less_leaves(cons(u, v), cons(w, z)) ->
  less_leaves(concat(u, v), concat(w, z))
 }
 Natural interpretation:
  Strict:
   {
                      concat(leaf(), y) -> y,
                  concat(cons(u, v), y) -> cons(u, concat(v, y)),
                 less_leaves(x, leaf()) -> false(),
        less_leaves(leaf(), cons(w, z)) -> true(),
    less_leaves(cons(u, v), cons(w, z)) ->
    less_leaves(concat(u, v), concat(w, z))
   }
  Weak:
   {}
  Interpretation class: deltarestricted
  [true](delta) = + 0 + 0*delta
  [less_leaves](delta, X1, X0) = + 0*X0 + 0*X1 + 0 + 1*X0*delta + 1*X1*delta + 0*delta
  [false](delta) = + 0 + 0*delta
  [cons](delta, X1, X0) = + 1*X0 + 0*X1 + 1 + 0*X0*delta + 2*X1*delta + 0*delta
  [leaf](delta) = + 0 + 2*delta
  [concat](delta, X1, X0) = + 1*X0 + 1*X1 + 0 + 0*X0*delta + 1*X1*delta + 0*delta
  
  less_leaves_tau_1(delta) = delta/(0 + 1 * delta)less_leaves_tau_2(delta) = delta/(0 + 1 * delta)
  
  cons_tau_1(delta) = delta/(0 + 2 * delta)cons_tau_2(delta) = delta/(1 + 0 * delta)
  
  concat_tau_1(delta) = delta/(1 + 1 * delta)concat_tau_2(delta) = delta/(1 + 0 * delta)
  
  Qed