'In the pl196x corpus each category is stored in single
file and thus both methods provide identical functionality. In order
to accommodate finer granularity, a non-standard textids() method was
implemented. All the main functions can be supplied with a list
of required chunks---giving much more control to the user.'

def textids(self, fileids=None, categories=None):

    (fileids, _) = self._resolve(fileids, categories)
    if (fileids is None):
        return sorted(self._t2f)
    if isinstance(fileids, string_types):
        fileids = [fileids]
    return sorted(sum((self._f2t[d] for d in fileids), []))
