(* 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_readonly: forall b lo hi m m'
(M: Mem.free m b lo hi = Some m'),
forall b, Mem.valid_block m b -> readonly m b m'.