def calls(self, call):
        """Redefine a call.

        The fake method will execute your function.  I.E.::

            >>> f = Fake().provides('hello').calls(lambda: 'Why, hello there')
            >>> f.hello()
            'Why, hello there'

        """
        exp = self._get_current_call()
        exp.call_replacement = call
        return self