code: def _add_element(self, element, parent_node):
          
          if element.tag == 'unit':
              element_node_id = element.attrib['id']+':'+element.attrib['type']
              node_layers = {self.ns, self.ns+':unit', self.ns+':'+element.attrib['type']}
          elif element.tag == 'connective':
              element_node_id = element.attrib['id']+':connective'
              node_layers = {self.ns, self.ns+':connective'}
          elif element.tag == 'discourse':
              element_node_id = 'discourse'
              node_layers = {self.ns}
          else:  # <modifier>
              element_node_id = element.getparent().attrib['id']+':'+element.tag
              node_layers = {self.ns, self.ns+':modifier'}
  
          self.add_node(element_node_id, layers=node_layers)
          self.add_edge(parent_node, element_node_id, layers={self.ns},
                        edge_type=EdgeTypes.dominance_relation)
  
          if element.text:
              if self.tokenize:
                  for token in element.text.split():
                      self._add_token(token, element_node_id)
              else:
                  element_text = sanitize_string(element.text)
                  self.node[element_node_id].update(
                      {'label': u"{0}: {1}...".format(element_node_id,
                                                     element_text[:20])})
  
  
          for child_element in element.iterchildren():
              self._add_element(child_element, element_node_id)
  
          if element.tail:  # tokens _after_ the </element> closes
              if self.tokenize:
                  for token in element.tail.split():
                      self._add_token(token, parent_node)
              else:
                  tail_text = sanitize_string(element.tail)
                  self.node[parent_node].update(
                      {'label': u"{0}: {1}...".format(parent_node,
                                                      tail_text[:20])})
docs: add an element (i.e. a unit/connective/discourse or modifier)
          to the docgraph.