(** * Relating two memory states. *)
(** ** Memory extensions *)
(**  A store [m2] extends a store [m1] if [m2] can be obtained from [m1] by relaxing the permissions of [m1] (for instance, allocating larger blocks) and replacing some of the [Vundef] values stored in [m1] by more defined values stored in [m2] at the same addresses. *)
Parameter extends: mem -> mem -> Prop.

Axiom free_parallel_extends:
forall m1 m2 b lo hi m1',
extends m1 m2 ->
free m1 b lo hi = Some m1' ->
exists m2',
free m2 b lo hi = Some m2'
/\ extends m1' m2'.