proposition: Lemma fset_nil : [fset[key] x in [::] : seq K] = fset0.
proof: Proof. by apply/fsetP=> x; rewrite in_fset_nil. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrnat eqtype ssrfun seq) mathcomp(choice path finset finfun fintype bigop)