code: def write_edge (self, node):
          
          attrs = {
              u"source": u"%d" % self.nodes[node["parent_url"]]["id"],
              u"target": u"%d" % node["id"],
          }
          self.xml_starttag(u"edge", attrs=attrs)
          self.xml_tag(u"label", node["label"])
          self.xml_starttag(u"data")
          if self.has_part("result"):
              self.xml_tag(u"valid", u"%d" % node["valid"])
          self.xml_endtag(u"data")
          self.xml_endtag(u"edge")
docs: Write one edge.