'Use ``self._sentences`` to construct a value for ``self._readings``.'

def _construct_readings(self):

    self._readings = {}
    for sid in sorted(self._sentences):
        sentence = self._sentences[sid]
        readings = self._get_readings(sentence)
        self._readings[sid] = dict([(('%s-r%s' % (sid, rid)), reading.simplify()) for (rid, reading) in enumerate(sorted(readings, key=str))])
