old_contents: 
  
  class Derivative:
      """docstring for Derivative"""
      def __init__(self, param):
          self.input = param
  
      def __validator(self):
          pass
  
new_contents: import re
  
  class Derivative:
      """docstring for Derivative"""
      def __init__(self, param):
          self.input = param
  
      def __validator(self, asd):
          pass
  
      def __check_for_var(self, polynom):
          f = re.compile('[/d]+')
          f.findall(polynom)
          
  
subject: add some logic