Proof with try tauto.
intros. rewrite Share.lub_rel_recompose in H.
generalize (Share.decompose_recompose (Share.lub a1 a2, Share.lub b1 b2));intro.
rewrite H in H0.
rewrite Share.lub_rel_recompose in H0.
rewrite Share.decompose_recompose in H0.
split;congruence.
Qed.

Lemma writable_share_right: forall sh, writable_share sh -> Share.unrel Share.Rsh sh = Share.top.