proposition: Fact joinC : commutative join.
proof: Proof. by move=> x y; apply: le_anti; rewrite !join_lel. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat choice seq) mathcomp(path fintype tuple bigop finset div prime finfun) mathcomp(finset)