code: def add_child_rule(self, child_rule):
          
          self._child_allowed(child_rule)
          child_node = self.node.add_child(rule_label=child_rule.class_label)
          return child_node.data
docs: Add a child path in the :class:`Flow` graph using the given 
          :class:`Rule` subclass.  This will create a new child :class:`Node` in
          the associated :class:`Flow` object's state graph with a new
          :class:`FlowNodeData` instance attached.
          
          The :class:`Rule` must be allowed at this stage of the flow according
          to the hierarchy of rules.
  
          :param child_rule: 
              :class:`Rule` class to add to the flow as a child of :class:`Node`
              that this object owns
          :returns: 
              ``FlowNodeData`` that was added