docs: def __add_coreference_chain_tiers(self, docgraph, body,
                                        min_chain_length=3):
          """
          Parameters
          ----------
          docgraph : DiscourseDocumentGraph
              the document graph from which the chains will be extracted
          body : etree._Element
              an etree representation of the <basic_body> element (and all its
              descendants) of the Exmaralda file
          min_chain_length : int
              don't add tiers for chains with less than N elements (default: 3)
  
          TODO: this method assumes that each pointing relation chains signifies
          a coreference chain.
          """
          E = self.E
  
          for i, chain in enumerate(get_pointing_chains(docgraph)):
              chain_tier = E('tier',
                             {'id': "TIE{}".format(self.tier_count),
                              'category': "chain", 'type': "t",
                              'display-name': "[coref-chain-{}]".format(i)})
              self.tier_count += 1
  
              chain_length = len(chain)
              if chain_length < min_chain_length:
                  continue  # ignore short chains
  
              for j, node_id in enumerate(chain):
                  span_node_ids = get_span(docgraph, node_id)
                  if span_node_ids:
                      start_id, end_id = self.__span2event(span_node_ids)
                      element_str = "chain_{0}: {1}/{2}".format(
                          i, chain_length-j, chain_length)
                      chain_tier.append(
                          E('event', {'start': "T{}".format(start_id),
                                      'end': "T{}".format(end_id)}, element_str))
              body.append(chain_tier)