unfold Couple in |- *; simpl in |- *.
simple induction 1; intros H1 H2.
elim (H1 false).
intros bb1; elim bb1.
intros HF.
change (EQ (Paire Vide (Sing A')) (Sing B)) in HF.
cut F.
simple induction 1.
apply (not_EQ_Vide_Sing A').
apply EQ_tran with B.
apply IN_Sing_EQ; apply IN_sound_right with (Paire Vide (Sing A'));
auto with zfc.
apply EQ_sym; apply IN_Sing_EQ;
apply IN_sound_right with (Paire Vide (Sing A'));
auto with zfc.
change (EQ (Paire Vide (Sing A')) (Paire Vide (Sing B')) -> EQ A' B') in |- *.
intros HP; cut (EQ (Sing A') (Sing B')).
intros; auto with zfc.
cut (IN (Sing A') (Paire Vide (Sing B'))).
intros HI; elim (Paire_IN Vide (Sing B') (Sing A') HI).
intros; cut F.
simple induction 1.
apply not_EQ_Sing_Vide with A'; assumption.
trivial with zfc.
apply IN_sound_right with (Paire Vide (Sing A')); auto with zfc.
Qed.
(* Here we cheat. It is easier to define the cartesian product using    *)
(* the type theoretical product, i.e. we here use non set-theoretical   *)
(* constructions. We could however use the usual definitions.           *)
Definition Prod (E E' : Ens) : Ens :=
match E, E' with
| sup A f, sup A' f' =>
sup _
(fun c : prod_t A A' =>
match c with
| pair_t a a' => Couple (f a) (f' a')
end)
end.
Hint Resolve Paire_sound_left Paire_sound_right: zfc.

Theorem Couple_sound_left :
forall A A' B : Ens, EQ A A' -> EQ (Couple A B) (Couple A' B).