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_unreadable_shares:
forall sh1 sh2 sh,
sepalg.join sh1 sh2 sh ->
~ readable_share sh1 ->
~ readable_share sh2 ->
~ readable_share sh.