text: import simpy
  import logging
  
  
  class Item:
      """
      Items represent flows within and between functions.
      An item is an input to or an output from a function.
      """
  
      def __init__(self, env: simpy.Environment, logger: logging.Logger,
                   systemModel: dict, item: dict):
  
          self.name = item['identity']['name']
          self.id = item['identity']['id']
          self.size = item['attributes']['size']
          self.type = "Item"
  
      def set_payload(self):
          pass
  
      def get_payload(self):
          pass
  
  
  class ControlAction(Item):
      """
      A controller may provide control actions to control some process and
      to enforce constraints on the behavior of the controlled process.
      """
  
      def __init__(self, env: simpy.Environment, logger: logging.Logger,
                   systemModel: dict, item: dict):
  
          super(ControlAction, self).__init__(env, logger, systemModel, item)
  
          self.type = "ControlAction"
  
  
  class Feedback(Item):
      """
      Process models may be updated in part by feedback used to observe the controlled process.
      """
  
      def __init__(self, env: simpy.Environment, logger: logging.Logger,
                   systemModel: dict, item: dict):
  
          super(Feedback, self).__init__(env, logger, systemModel, item)
  
          self.type = "Feedback"
  
  
  class Context(Item):
      """
      The set of process model variables and values.
      """
  
      def __init__(self, env: simpy.Environment, logger: logging.Logger,
                   systemModel: dict, item: dict):
  
          super(Context, self).__init__(env, logger, systemModel, item)
  
          self.type = "Context"
  