proposition: Lemma fsetKUidC A B : A `|` (B `|` A) = B `|` A.
proof: Proof. by rewrite fsetUCA fsetUid. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrnat eqtype ssrfun seq) mathcomp(choice path finset finfun fintype bigop)