proposition: Lemma fsetUA A B C : A `|` (B `|` C) = A `|` B `|` C.
proof: Proof. by apply/fsetP => x; rewrite !inE orbA. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrnat eqtype ssrfun seq) mathcomp(choice path finset finfun fintype bigop)