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