intro; unfold iff in |- *; intro H; elim H; intros.
absurd (Some y = None); auto.
intro D; discriminate D.
Qed.

(* Specifications: *)
Lemma Chsubsc_Corr :
forall (s : SFSstate) (u : SUBJECT) (sc : SecClass),
FuncPre7 s ->
(fSSC (subjectSC s) u = None <-> fSSC (chsubsc_SC s u sc) u = None).