code: def derivative(self, point):
          
          return ZeroOperator(domain=self.domain, range=self.range)
docs: 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.])