Multiple Selection and Add/Delete Components using Custom Hook

Shows the use of a custom hook to factor out the common code when handling cities and selCities.

Bug

Assuming that we would like the invariant selCities ⊆ cities to be true, then we have a bug:

If a city like Chicago is added to Selected Cities and then removed from the list of cities using Do Remove, then it is impossible to remove from Selected Cities without adding it back to cities using Do Add.

App