Proof with auto.
intros.
icase j1; icase j2; icase j3.
red in H; red.
apply join_comm...
Qed.

Lemma ijoin_assoc {A} {JA: Join A}{PA: Perm_alg A}{SA: Sep_alg A}{CA: Disj_alg A} : forall a b c d e,
ijoin a b d ->
ijoin d c e ->
{f : ijoinable A | ijoin b c f /\ ijoin a f e}.