Since the usefulness of this totally depends what sort of district you have, it defaults to NULL, and it's expected that you'll define it when adding districts to the table. The motivating example is to derive a Borough from a City Council district.
… nosetests. Also, Ian likes [pipeline] better than 'filter-with'.
…s Borough President now.)
…need disambiguation later when we care about multiple states). Also factored out a few repeated bits of code.