code: def downstream(self, node):
          
          graph = self.graph
          if node not in graph:
              raise KeyError('node %s is not in graph' % node)
          return list(graph[node])
docs: Returns a list of all nodes this node has edges towards.
  
          Args:
              node (str): The node whose downstream nodes you want to find.
  
          Returns:
              list: A list of nodes that are immediately downstream from the
                    node.