code: def __add_annotation_tier(self, docgraph, body, annotation_layer):
          
          layer_cat = annotation_layer.split(':')[-1]
          temp_tier = self.E('tier',
                             {'id': "TIE{}".format(self.tier_count),
                              'category': layer_cat, 'type': "t",
                              'display-name': "[{}]".format(annotation_layer)})
          self.tier_count += 1
  
          for node_id in select_nodes_by_layer(docgraph, annotation_layer):
              span_node_ids = get_span(docgraph, node_id)
              if span_node_ids:
                  start_id, end_id = self.__span2event(span_node_ids)
                  event_label = docgraph.node[node_id].get('label', '')
                  event = self.E('event',
                                 {'start': "T{}".format(start_id),
                                  'end': "T{}".format(end_id)},
                                 event_label)
                  temp_tier.append(event)
          body.append(temp_tier)
docs: adds a span-based annotation layer as a <tier> to the Exmaralda <body>.
  
          Parameter
          ---------
          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
          annotation_layer : str
              the name of a layer, e.g. 'tiger', 'tiger:token' or 'mmax:sentence'