intros.
elim H; intros; elim H3; intros; elim H4; intros; elim H5; intros.
elim H7; intros; elim H9; intros; elim H11; intros; elim (H15 x H0); intros.
elim H17; intros; clear H2 H3 H5 H6 H7 H9 H10 H11 H12 H13 H14 H15 H16 H17 H19.
rewrite (mult_opp_r H (Opp x) y H18 H1).
rewrite (mult_opp_l H x y H0 H1).
symmetry  in |- *.
exact (opp_opp S A Add O Opp H4 (Mult x y) (H8 x y H0 H1)).
Qed.
(******************)
Definition integrity := forall a b : S, Mult a b = O -> {a = O} + {b = O}.
(************************************)
Definition is_unitary_commutative_ring :=
is_ring /\ commutativity S Mult /\ neutral S A Mult I.
(* Division *)
(***************)
Definition divide (a b : S) :=
A a /\ A b /\ (b = O \/ a <> O /\ (exists q : S, A q /\ b = Mult a q)).

(* Specifications: *)
(**************)
Theorem div_O_O : is_ring -> divide O O.
unfold divide in |- *; intros.