code: def derivative(self, x):
          
          if self.is_linear:
              return self
          else:
              return OperatorSum(self.left.derivative(x),
                                 self.right.derivative(x),
                                 self.__tmp_dom, self.__tmp_ran)
docs: Return the operator derivative at ``x``.
  
          The derivative of a sum of two operators is equal to the sum of
          the derivatives.
  
          Parameters
          ----------
          x : `domain` `element-like`
              Evaluation point of the derivative