docs: def __add_token_tiers(self, docgraph, body):
          """
          adds all tiers that annotate single tokens (e.g. token string, lemma,
          POS tag) to the etree representation of the Exmaralda XML file.
  
          Parameters
          ----------
          docgraph : DiscourseDocumentGraph
              the document graph to be converted
          body : etree._Element
              an etree representation of the <basic_body> element (and all its
              descendants) of the Exmaralda file
          """
          E = self.E
          token_tier = E('tier',
                         {'id': "TIE{}".format(self.tier_count),
                          'category': "tok", 'type': "t",
                          'display-name': "[tok]"})
          self.tier_count += 1
  
          token_attribs = defaultdict(lambda: defaultdict(str))
          for token_node_id in docgraph.tokens:
              for attrib in docgraph.node[token_node_id]:
                  is_boring_attrib = attrib in ('layers', 'label')
                  is_boring_cat = attrib.split(':')[-1] in ('token',
                                                            'id', 'word',
                                                            'morph', 'lemma')
                  if not is_boring_attrib and not is_boring_cat:
                      token_attribs[attrib][token_node_id] = \
                          docgraph.node[token_node_id][attrib]
  
          for i, (_tok_id, token_str) in enumerate(docgraph.get_tokens()):
              # example: <event start="T0" end="T1">Zum</event>
              token_tier.append(
                  E('event', {'start': "T{}".format(i),
                              'end': "T{}".format(i+1)}, token_str))
          body.append(token_tier)
  
          for anno_tier in token_attribs:
              category = anno_tier.split(':')[-1]
              temp_tier = E(
                  'tier', {'id': "TIE{}".format(self.tier_count),
                           'category': category, 'type': "t",
                           'display-name': "[{}]".format(anno_tier)})
              self.tier_count += 1
              for token_node_id in token_attribs[anno_tier]:
                  token_tier_id = self.toknode2id[token_node_id]
                  token_attrib = token_attribs[anno_tier][token_node_id]
                  temp_tier.append(
                      E('event', {'start': "T{}".format(token_tier_id),
                                  'end': "T{}".format(token_tier_id+1)},
                        token_attrib))
              body.append(temp_tier)
          return body