proposition: Lemma joinxB x y z : x `|` (y `\` z) = ((x `|` y) `\` z) `|` (x `&` z).
proof: Proof. by rewrite diffUx joinAC joinBI. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat choice seq) mathcomp(path fintype tuple bigop finset div prime finfun) mathcomp(finset)