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 glb_Rsh_not_top: forall sh, Share.glb Share.Rsh sh <> Share.top.