proposition: Lemma catf0 f : f + [fmap] = f.
proof: Proof. by apply/fmapP => k; rewrite fnd_cat in_fset0. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrnat eqtype ssrfun seq) mathcomp(choice path finset finfun fintype bigop)