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.

def add_callback(self, f):
        """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.
        """
        self.callback = f
        return self.decorate(self.fref())