@staticmethod
def _mark_coreference(word, coreference_string):
    """ Append to a word a coreference string
        :param word: The word that forms part of a mention
        :param coreference_string: The coreference string
        """
    if 'coreference' not in word:
        word['coreference'] = [coreference_string]
    else:
        word['coreference'].append(coreference_string)
