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 sepcon_derives {A} {JA: Join A}{PA: Perm_alg A}: forall p q p' q', (p |-- p') -> (q |-- q') -> (p * q |-- p' * q').