code: def on_session_destroyed(self, *callbacks):
          ''' Provide callbacks to invoke when the session serving the Document
          is destroyed
  
          '''
          for callback in callbacks:
              _check_callback(callback, ('session_context',))
              self._session_destroyed_callbacks.add(callbackf on_session_destroyed(self, *callbacks):
          ''' Provide callbacks to invoke when the session serving the Document
          is destroyed
  
          '''
          for callback in callbacks:
              _check_callback(callback, ('session_context',))
              self._session_destroyed_callbacks.add(callback)
docs: Provide callbacks to invoke when the session serving the Document
          is destroyed