(** [perm m b ofs k p] holds if the address [b, ofs] in memory state [m] has permission [p]: one of freeable, writable, readable, and nonempty. If the address is empty, [perm m b ofs p] is false for all values of [p]. [k] is the kind of permission we are interested in: either the current permissions or the maximal permissions. *)
Parameter perm: forall (m: mem) (b: block) (ofs: Z) (k: perm_kind) (p: permission), Prop.
(** Logical implications between permissions *)

Axiom perm_implies:
forall m b ofs k p1 p2, perm m b ofs k p1 -> perm_order p1 p2 -> perm m b ofs k p2.