def _read_coreference(corpus_soup):
    """ Obtain all mentions and coreference clusters in current document.

    Returns
    -------
    tuple:
        (dict[str, list[str]], list[list[str]]):
            (1.) mentions
            (2.) mentions organized by coreference cluster
    """
    mentions = {}
    clusters = []
    for cluster_obj in corpus_soup.findAll("tc:entity"):
        curr_cluster = []
        for mention_obj in cluster_obj.findAll("tc:reference"):
            mention_id = mention_obj["id"]
            mention_tokens = mention_obj["tokenids"].split(" ")
            mentions[mention_id] = mention_tokens
            curr_cluster.append(mention_id)

        clusters.append(curr_cluster)
    return mentions, clusters