old_contents: from __future__ import absolute_import, print_function, division
  
  
  class Derivative(object):
      "Abstract symbolic object for a derivative."
  
      def __init__(self, name, doc):
          self.__doc__ = doc
          self.name = name
  
      def __str__(self):
          return self.name
  
  grad = Derivative("grad", "Symbol for the gradient operation")
  div = Derivative("div", "Symbol for the divergence operation")
  curl = Derivative("curl", "Symbol for the curl operation")
  
new_contents: from __future__ import absolute_import, print_function, division
  
  
  class Derivative(object):
      "Abstract symbolic object for a derivative."
  
      def __init__(self, name, doc):
          self.__doc__ = doc
          self.name = name
  
      def __str__(self):
          return self.name
  
  
  grad = Derivative("grad", "Symbol for the gradient operation")
  div = Derivative("div", "Symbol for the divergence operation")
  curl = Derivative("curl", "Symbol for the curl operation")
  
subject: fix latest flake8 issue