code: def connect_child(self, node):
          
          if node.graph != self.graph:
              raise AttributeError('cannot connect nodes from different graphs')
  
          node.parents.add(self)
          self.children.add(node)
docs: Adds the given node as a child to this one.  No new nodes are
          created, only connections are made.
          
          :param node:
              a ``Node`` object to connect