private static Function<?, ?> addStateToCity(String city, String state) {

	// make sure that the combination of city/state is unique
	// as there may be multiple zipcode records for a city

	CityStates cityInStates = cityStates.get(city);

	if (cityInStates == null) {
		cityStates.put(city,  new CityStates(city, state));
	} else {
		if (!cityInStates.statesContainsState(state)) {
			cityInStates.addState(state);
			cityStates.put(city, cityInStates);
		}
	}
	return null;
}