(*+ Collection of disjoint maps *)

(** For the proof of composition of safety, we need to maintain the
    invariants "all the maps are disjoint*" which is why we need this
    section.

  * pairwise-disjointness is equivalent to arbitrary disjointness
    because the permissions are "set-theoretical" *)

Definition selfjoins {A} (maps : A -> map) :=
  forall i j, i <> j -> joins (maps i) (maps j).Definition sameperms (m n : map) := forall x, m x = None <-> n x = None.

Lemma sameperms_sym m n : sameperms m n -> sameperms n m.