Columns of xpaths.csv whose values are computed from the filing
evidence by update_xpath_versions() (and the v1 columns they replaced).
The change log records when one is added or removed, not its cell values:
they are measurements, rebuilt from the evidence, and logging each refresh
would add a row per xpath. diff_src() skips their cells,
check_changelog() does not require them to be logged, and they can be
removed while they still hold values.