proposition: Lemma FSetK A (X : {set A}) : fsub A [fsetval k in X] = X.
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)