intros A B C l1 l2; elim l2; simpl in |- *; auto.
intros a H' H'0; elim H'0.
intros a l H' a0 b H'0.
case (in_app_or _ _ _ H'0); auto with datatypes.
elim l1; simpl in |- *; auto with datatypes.
intros a1 l0 H'1 H'2; elim H'2; clear H'2; intros H'3;
[ inversion H'3 | idtac ]; auto with datatypes.
intros H'1; apply H' with (b := b); auto.
Qed.

Theorem mProd_correct_rev2 :
forall (A B C : Set) (l1 : list A) (l2 : list B) (a : A) (b : B),
In (a, b) (mProd A B C l1 l2) -> In b l2.