proposition: Lemma fsetUCA A B C : A `|` (B `|` C) = B `|` (A `|` C).
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)