Definition wk_split {A} {JA: Join A} :=
forall a b c d e : A, join a b c -> join d e c -> joins a d -> join_sub d b.

Lemma ewand_derives {A} {JA: Join A}{PA: Perm_alg A}{AG: ageable A}{XA: Age_alg A}:
forall P P' Q Q',  P |-- P' -> Q |-- Q' -> ewand P Q |-- ewand P' Q'.