intros A B C l1 l2; elim l2; simpl in |- *; auto.
intros a l H' a0 b H'0 H'1; elim H'1;
[ intros H'2; rewrite <- H'2; clear H'1 | intros H'2; clear H'1 ];
auto with datatypes.
apply in_or_app; left; auto with datatypes.
generalize H'0; elim l1; simpl in |- *; auto with datatypes.
intros a1 l0 H'1 H'3; elim H'3; clear H'3; intros H'4;
[ rewrite <- H'4 | idtac ]; auto with datatypes.
Qed.

Theorem mProd_correct_rev1 :
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 a l1.