code: def write_edge (self, node):
          
          self.writeln(u"  edge [")
          self.writeln(u'    label  "%s"' % node["edge"])
          self.writeln(u"    source %d" % self.nodes[node["parent_url"]]["id"])
          self.writeln(u"    target %d" % node["id"])
          if self.has_part("result"):
              self.writeln(u"    valid  %d" % node["valid"])
          self.writeln(u"  ]")
docs: Write one edge.