def METHOD_NAME():
    mrts_before = timeseries.MultiRegionDataset.from_csv(
        io.StringIO(
            "location_id,date,cases\n"
            "iso1:us#fips:97111,2020-01-01,100\n"
            "iso1:us#fips:97111,2020-01-02,\n"
            "iso1:us#fips:97111,2020-01-03,110\n"
            "iso1:us#fips:97111,2020-01-04,130\n"
        )
    )
    mrts_expected = timeseries.MultiRegionDataset.from_csv(
        io.StringIO(
            "location_id,date,cases,new_cases\n"
            "iso1:us#fips:97111,2020-01-01,100,\n"
            "iso1:us#fips:97111,2020-01-02,,\n"
            "iso1:us#fips:97111,2020-01-03,110,\n"
            "iso1:us#fips:97111,2020-01-04,130,20\n"
        )
    )
    timeseries_after = new_cases_and_deaths.add_new_cases(mrts_before)
    test_helpers.assert_dataset_like(mrts_expected, timeseries_after)