Debugging dump of the current subtoken vocabulary.

def dump(self):
    """Debugging dump of the current subtoken vocabulary."""
    subtoken_strings = [(i, s)
                        for s, i in six.iteritems(self._subtoken_string_to_id)]
    print(u", ".join(u"{0} : '{1}'".format(i, s)
                     for i, s in sorted(subtoken_strings)))