code: def del_edge(self, edge, double=False):
          
          (left_entity, right_entity) = edge
          self[left_entity].pop(right_entity)
          if double:
              self.del_edge((edge[1], edge[0]))
docs: Removes an edge from the ``DictGraph``. An edge is a pair of **node 
          objects**. The **node objects** are not removed from the ``DictGraph``.
          
          Arguments:
          
            - edge(``tuple``) An ordered pair of **node objects**. The edge is 
              assumed to have a direction from the first to the second **node 
              object**.
            - double(``bool``) [default: ``False```] If ``True`` the the reverse
              edge is also removed.