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