(** * 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 store_within_extends:
forall chunk m1 m2 b ofs v1 m1' v2,
extends m1 m2 ->
store chunk m1 b ofs v1 = Some m1' ->
Val.lessdef v1 v2 ->
exists m2',
store chunk m2 b ofs v2 = Some m2'
/\ extends m1' m2'.