proposition: Lemma fset_sub A (P : pred K) : [fset x in A | P x] `<=` A.
proof: Proof. by apply/fsubsetP => k; rewrite in_fset inE /= => /andP []. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrnat eqtype ssrfun seq) mathcomp(choice path finset finfun fintype bigop)