Create a new coreference object and add it to the coreferences layer
        @type coref_type: string
        @param coref_type: type of the coreference object
        @type term_ids: list
        @param term_ids: list of term ids
        @type id: string
        @param id: optional id of the entity

def create_coreference(self, coref_type, term_ids, id=None):
        """
        Create a new coreference object and add it to the coreferences layer
        @type coref_type: string
        @param coref_type: type of the coreference object
        @type term_ids: list
        @param term_ids: list of term ids
        @type id: string
        @param id: optional id of the entity
        """
        if id is None:
            if self.coreference_layer is None:
                i = 1
            else:
                corefs = (l for l in self.coreference_layer.get_corefs() if l.get_type == coref_type)
                i = len(list(corefs)) + 1
            id = "co{coref_type}{i}".format(**locals())

        new_coref = Ccoreference(type=self.type)
        new_coref.set_id(id)
        new_coref.set_type(coref_type)
        new_coref.add_span(term_ids)
        self.add_coreference(new_coref)
        return new_coref