(** * Validity of blocks and accesses *)

(** A block address is valid if it was previously allocated. It remains valid
  even after being freed. *)

Definition valid_block (m: mem) (b: block) := Plt b (nextblock m).

Theorem 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.