proposition: Lemma fsetUC (A B : {fset K}) : A `|` B = B `|` A.
proof: Proof. by apply/fsetP => a; rewrite !inE orbC. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrnat eqtype ssrfun seq) mathcomp(choice path finset finfun fintype bigop)