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