(* Lemma unchanged_on_sym P m m' b: Mem.unchanged_on P m m' -> Mem.valid_block m b -> Mem.unchanged_on (fun bb z => P bb z /\ bb=b) m' m. *)

(* Lemma free_forward: forall b z0 z m m' (M: Mem.free m b z0 z = Some m'), mem_forward m m'. *)
Lemma free_forward: forall b z0 z m m'
(M: Mem.free m b z0 z = Some m'),
mem_forward m m'.