proposition: Lemma itv_joinA : associative itv_join.
proof: Proof. by case=> [? ?][? ?][? ?] /=; rewrite meetA joinA. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq choice) mathcomp(div fintype bigop order ssralg finset fingroup) mathcomp(ssrnum)