proposition: Lemma fset0D A : fset0 `\` A = fset0.
proof: Proof. by apply/fsetP=> x; rewrite !inE andbF. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrnat eqtype ssrfun seq) mathcomp(choice path finset finfun fintype bigop)