(* not useful? *)

Lemma field_at_share_join_W {cs: compspecs}:
forall sh1 sh2 sh t gfs v1 v2 p,
sepalg.join sh1 sh2 sh ->
writable_share sh1 ->
field_at sh1 t gfs v1 p * field_at sh2 t gfs v2 p |-- field_at sh t gfs v1 p.