Hint Resolve writable_readable_share.
Definition extern_retainer := fst (Share.split Share.Lsh).
Definition Ews (* extern_write_share *) :=
Share.lub extern_retainer Share.Rsh.

Lemma join_readable2:
forall sh1 sh2 sh3,
join sh1 sh2 sh3 ->
readable_share sh2 -> readable_share sh3.