proposition: Lemma fsetU1K a B : a \notin B -> (a |` B) `\ a = B.
proof: Proof. by move/negPf=> nBa; apply/fsetP=> x; rewrite !inE; case: eqP => // ->. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrnat eqtype ssrfun seq) mathcomp(choice path finset finfun fintype bigop)