proposition: Lemma fsetD1K a A : a \in A -> a |` (A `\ a) = A.
proof: Proof. by move=> Aa; 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)