what would you like to build in this city?

[Blaupanzer]

If in the very strange but apparently possible situation of building units to delete them, modify that a bit by building the most up-to date units, then delete to oldest most obsolete units. Again, even at marathon speed, seems unlikely that every city would have every building available to it prior to your reaching writing. After the libraries, if no other building remains for a city and your unit numbers have your costs equal to your income, then build beakers.