'Initialize the indexes ``_lemma_to_class``,
``_wordnet_to_class``, and ``_class_to_fileid`` by scanning
through the corpus fileids.  This is fast with cElementTree
(<0.1 secs), but quite slow (>10 secs) with the python
implementation of ElementTree.'

def _index(self):

    for fileid in self._fileids:
        self._index_helper(self.xml(fileid), fileid)
