(** Memory states that inject into themselves. *)
Definition flat_inj (thr: block) : meminj :=
fun (b: block) => if plt b thr then Some(b, 0) else None.
Parameter inject_neutral: forall (thr: block) (m: mem), Prop.

Axiom drop_inject_neutral:
forall m b lo hi p m' thr,
drop_perm m b lo hi p = Some m' ->
inject_neutral thr m ->
Plt b thr ->
inject_neutral thr m'.