code: def add_relation(self, relation):
          
          if self.ignore_relations is False:
              parent_node_id = self.get_parent_id(relation)
              reltype = relation.attrib['type']
              # add relation type information to parent node
              self.node[parent_node_id].update({'relation': reltype})
              self.add_layer(parent_node_id, self.ns+':'+reltype)
              if 'target' in relation.attrib:
                  # if the relation has no target, it is either 'expletive' or
                  # 'inherent_reflexive', both of which should not be part of the
                  # 'markable' layer
                  self.add_layer(parent_node_id, self.ns+':markable')
                  target_id = relation.attrib['target']
                  self.add_edge(parent_node_id, target_id,
                                layers={self.ns, self.ns+':'+reltype,
                                        self.ns+':coreference'},
                                label=reltype,
                                edge_type=dg.EdgeTypes.pointing_relation)
                  self.add_layer(target_id, self.ns+':markable')
docs: Parameters
          ----------
          relation : etree.Element
              etree representation of a <relation> element
              A <relation> always has a type attribute and inherits
              its ID from its parent element. In the case of a non-expletive
              relation, it also has a target attribute.
  
          Example
          -------
  
            <node xml:id="s29_501" cat="NX" func="ON" parent="s29_523">
             <relation type="expletive"/>
             <word xml:id="s29_2" form="es" pos="PPER" morph="nsn3" lemma="es"
                   func="HD" parent="s29_501" dephead="s29_14" deprel="SUBJ"/>
            </node>
  
            ...
  
           <node xml:id="s4_507" cat="NX" func="ON" parent="s4_513">
            <relation type="coreferential" target="s1_502"/>
            <node xml:id="s4_505" cat="NX" func="HD" parent="s4_507">
            ...
            </node>
           </node>