proposition: Lemma IInv_strext : forall x y : subset I, [--] (f x) [#] [--] (f y) -> x [#] y.
proof: Proof. intros x y H. generalize (un_op_strext_unfolded _ _ _ _ H); intro H0. exact (csf_strext_unfolded _ _ _ _ _ H0). Qed.
imports: CoRN.ftc.PartFunEquality