code: def write_edge (self, node):
          
          source = dotquote(self.nodes[node["parent_url"]]["label"])
          target = dotquote(node["label"])
          self.writeln(u'  "%s" -> "%s" [' % (source, target))
          self.writeln(u'    label="%s",' % dotquote(node["edge"]))
          if self.has_part("result"):
              self.writeln(u"    valid=%d," % node["valid"])
          self.writeln(u"  ];")
docs: Write edge from parent to node.