Definition cleave (sh: share) :=
  (Share.lub (fst (Share.split (Share.glb Share.Lsh sh))) (fst (Share.split (Share.glb Share.Rsh sh))),
   Share.lub (snd (Share.split (Share.glb Share.Lsh sh))) (snd (Share.split (Share.glb Share.Rsh sh)))).

Lemma cleave_readable1: forall sh, readable_share sh -> readable_share (fst (cleave sh)).