Section Reference.
(* One common kind of PCM is one in which a central authority has a reference copy, and clients pass around partial knowledge. When a client recovers all pieces, it can gain full knowledge. *)
(* This is related to the snapshot PCM, but the snapshots aren't duplicable. *)

Lemma join_Bot : forall a b, sepalg.join a b Share.bot -> a = Share.bot /\ b = Share.bot.