proposition: Lemma fset_cat s s' : [fset[key] x in s ++ s'] = [fset[key] x in s] `|` [fset[key] x in s'].
proof: Proof. by apply/fsetP=> x; rewrite !inE !in_fset_cat. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrnat eqtype ssrfun seq) mathcomp(choice path finset finfun fintype bigop)