Definition Tsh : share := Share.top.
Definition nonempty_share (sh: share) :=
sepalg.nonidentity sh.
Definition readable_share (sh: share) :=
nonempty_share (Share.glb Share.Rsh sh).
Definition writable_share (sh: share) :=
nonempty_share (Share.glb Share.Lsh sh) /\ join_sub Share.Rsh sh.

Lemma readable_share_dec:
forall sh, {readable_share sh}+{~ readable_share sh}.