Definition pow_N_gen (R:Type)(x1:R)(m:R->R->R)(x:R) (p:N) :=
  match p with
  | N0 => x1
  | Npos p => pow_pos_gen m x p
  end.

Lemma Ppow_N_ok : forall l, (forall P, subst_l P@l == P@l) -> forall P n, (Ppow_N P n)@l == (pow_N_gen P1 Pmul P n)@l.