code: def on_change(self, *callbacks):
          ''' Provide callbacks to invoke if the document or any Model reachable
          from its roots changes.
  
          '''
          for callback in callbacks:
  
              if callback in self._callbacks: continue
  
              _check_callback(callback, ('event',))
  
              self._callbacks[callback] = callbacf on_change(self, *callbacks):
          ''' Provide callbacks to invoke if the document or any Model reachable
          from its roots changes.
  
          '''
          for callback in callbacks:
  
              if callback in self._callbacks: continue
  
              _check_callback(callback, ('event',))
  
              self._callbacks[callback] = callback
docs: Provide callbacks to invoke if the document or any Model reachable
          from its roots changes.