intros a2; elim (e2 a2); intros a1 H1; exists a1; auto.
intros a1; elim (e1 a1); intros a2 H2; exists a2; auto.
Qed.

Theorem EQ_INC : forall E E' : Ens, EQ E E' -> INC E E'.
simple induction E; intros A f r; simple induction E'; intros A' f' r';
simpl in |- *; simple induction 1; intros e1 e2; unfold INC in |- *;
simpl in |- *.