proposition: Lemma imfset_id (A : {fset K}) : id @` A = A.
proof: Proof. by apply/fsetP=> a; rewrite in_fset. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrnat eqtype ssrfun seq) mathcomp(choice path finset finfun fintype bigop)