The input copy is thus treated as a canonical text. Potential violations of
the sub-conditions a-e are tested at successively higher levels of the input
copy: constituent, sentence, story; then other available copy, and real-world
knowledge. The basic information unit of news is the story rather than the
sentence. Apparent inaccuracies at the level of the sentence are frequent,
but usually they merely increase or reduce redundancy, or incorporate
information from other sentences of the input. Sometimes a change which
may still appear inaccurate in the context of the input story is warranted by
information in other wires on the same topic. On rare occasions, we must
seek a warrant in sources beyond the news, for instance to check on
geography.