simple induction E''; unfold INC in |- *; simpl in |- *;
intros A f HR e H1 E0 i; apply H1.
apply IN_sound_right with E'; auto with zfc.
Qed.

(* Specifications: *)
Theorem INC_sound_right :
forall E E' E'' : Ens, EQ E' E'' -> INC E E' -> INC E E''.
Theorem INC_sound_right :
forall E E' E'' : Ens, EQ E' E'' -> INC E E' -> INC E E''.