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