proposition: Lemma fpowerset1 (x : K) : fpowerset [fset x] = [fset fset0; [fset x]].
proof: Proof. by apply/fsetP => X; rewrite !inE fpowersetE fsubset1 orbC. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrnat eqtype ssrfun seq) mathcomp(choice path finset finfun fintype bigop)