'Remove a sentence from the current discourse.
Updates ``self._input``, ``self._sentences`` and ``self._readings``.
:param sentence: An input sentence
:type sentence: str
:param verbose: If ``True``,  report on the updated list of sentences.'

def retract_sentence(self, sentence, verbose=True):

    try:
        self._input.remove(sentence)
    except ValueError:
        print(("Retraction    failed.    The    sentence    '%s'    is    not    part    of    the    current    discourse:" % sentence))
        self.sentences()
        return None
    self._sentences = dict([(('s%s' % i), sent) for (i, sent) in enumerate(self._input)])
    self.readings(verbose=False)
    if verbose:
        print('Current    sentences    are    ')
        self.sentences()
