Inductive not_racy : option permission -> Prop :=
| empty : not_racy None.
Inductive racy : option permission -> Prop :=
| freeable : racy (Some Freeable).

Lemma perm_order_clash:
forall p p'
(Hreadable: Mem.perm_order' p Readable)
(Hwritable: Mem.perm_order' p' Writable),
~ exists pu, perm_union p p' = Some pu.