code: def _data_graph_add_edge(self, src, dst, **edge_labels):
          
  
          if src in self._data_graph and dst in self._data_graph[src]:
              return
  
          self._data_graph.add_edge(src, dst, **edge_labels)
  
          self._simplified_data_graph = None
docs: Add an edge in the data dependence graph.
  
          :param ProgramVariable src: Source node.
          :param ProgramVariable dst: Destination node.
          :param edge_labels: All labels associated with the edge.
          :return: None