export const useNavigator = (): Service => {
  const nav = React.useContext(NavigatorReactContext);
  if (nav === null) {
    throw new Error("Navigator: render inside View.compose(…).Provider (or Navigator.Provider)");
  }
  const [, bump] = React.useReducer((n: number) => n + 1, 0);
  React.useEffect(() => nav.subscribe(bump), [nav]);
  return nav;
};