old_contents: from django.dispatch import Signal
  
  message_handled = Signal(
      providing_args=['message', 'instance', 'new_revision', 'transition_result']
      )
  
new_contents: from django.dispatch import Signal
  
  message_handled = Signal(
      providing_args=[
          'message',
          'instance',
          'new_instance',
          'new_revision',
          'transition_result'
      ])
  
  transition_handled = Signal(
      providing_args=[
          'message',
          'instance',
          'new_instance',
          'new_revision',
          'transition_result'
      ])
  
subject: Add signal: transition_handled