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