Definition permDisjoint p1 p2:=
    exists pu : option permission,
      perm_union p1 p2 = Some pu.

(* Specifications: *)
Lemma permDisjoint_None: forall p, permDisjoint None p.
Lemma permDisjoint_comm: forall p1 p2, permDisjoint p1 p2 -> permDisjoint p2 p1.
Lemma permDisjointLT: forall a b c, permDisjoint a c -> Mem.perm_order'' a b -> permDisjoint b c.