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 exists_sepcon1 {A} {JA: Join A}{PA: Perm_alg A}: forall T (P: T -> pred A) Q, exp P * Q = exp (fun x => P x * Q).