code: def add_callback(self, f):
          
          self.callback = f
          return self.decorate(self.fref())
docs: Magic method assigned to f.callback that allows a callback to be
          defined as follows:
  
          @bucket
          def fun(...):
              ...
  
          @fun.callback
          def fun():
              ...
  
          In the event that a cached result is used, the callback is fired.