def derivative(self, point):
        """Derivative of this operator, always zero.

        Returns
        -------
        derivative : `ZeroOperator`

        Examples
        --------
        >>> r3 = odl.rn(3)
        >>> x = r3.element([1, 2, 3])
        >>> op = ConstantOperator(x)
        >>> deriv = op.derivative([1, 1, 1])
        >>> deriv([2, 2, 2])
        rn(3).element([ 0.,  0.,  0.])
        """
        return ZeroOperator(domain=self.domain, range=self.range)