Definition ewand {A} {JA: Join A} (P Q: pred A) : pred A :=
  fun w => exists w1, exists w2, join w1 w w2 /\ P w1 /\ Q w2.

Lemma exp_derives {A} : forall B (P: B -> pred A) Q , (forall x:B, P x |-- Q x) -> (exp P |-- exp Q).