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