intros f p q r s x y H.
destruct H; rewrite H.
rewrite plus_0_l.
unfold partial_derive.
simpl.
rewrite -Derive_n_comp.
now apply Derive_n_ext.
rewrite plus_0_r.
unfold partial_derive.
simpl.
apply Derive_n_ext.
intros y0.
rewrite -Derive_n_comp.
now apply Derive_n_ext.
Qed.
(** * Iterated differential *)
Fixpoint ex_diff_n f n x y :=
continuity_2d_pt f x y /\
match n with
| O => True
| S n =>
ex_derive (fun z => f z y) x /\
ex_derive (fun z => f x z) y /\
ex_diff_n (fun u v => Derive (fun z => f z v) u) n x y /\
ex_diff_n (fun u v => Derive (fun z => f u z) v) n x y
end.

Lemma ex_diff_n_m :
forall n m, (m <= n)%nat -> forall f x y, ex_diff_n f n x y -> ex_diff_n f m x y.