code: def connect_child(self, child_node):
          
          self._child_allowed(child_node.rule)
          self.node.connect_child(child_node.node)
docs: Adds a connection to an existing rule in the :class`Flow` graph.
          The given :class`Rule` subclass must be allowed to be connected at
          this stage of the flow according to the hierarchy of rules.
  
          :param child_node: 
              ``FlowNodeData`` to attach as a child