code: def add_edge(self, graph_edge):
          
  
          if not isinstance(graph_edge, Edge):
              raise TypeError('add_edge() received a non edge class object: ' + str(graph_edge))
  
          edge_points = ( graph_edge.get_source(), graph_edge.get_destination() )
  
          if self.obj_dict['edges'].has_key(edge_points):
  
              edge_list = self.obj_dict['edges'][edge_points]
              edge_list.append(graph_edge.obj_dict)
  
          else:
  
              self.obj_dict['edges'][edge_points] = [ graph_edge.obj_dict ]
  
  
          graph_edge.set_sequence( self.get_next_sequence_number() )
  
          graph_edge.set_parent_graph( self.get_parent_graph() )
docs: Adds an edge object to the graph.
          
          It takes a edge object as its only argument and returns
          None.