Definition PMap_eq {A} (m m' : PMap.t (Z -> A)) := forall b' ofs', m !! b' ofs' = m' !! b' ofs'.

Lemma setPerm_comm b1 o1 b2 o2 e:
PMap_eq (setPerm (Some Writable) b1 o1 (setPerm (Some Writable) b2 o2 e))
(setPerm (Some Writable) b2 o2 (setPerm (Some Writable) b1 o1 e)).