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

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