docs: def unregister(self, observers):
          u"""
          Concrete method of Subject.unregister().
          Unregister observers as an argument to self.observers.
          """
  
          if isinstance(observers, list) or isinstance(observers, tuple):
              for observer in observers:
                  try:
                      index = self._observers.index(observer)
                      self._observers.remove(self._observers[index])
                  except ValueError:
                      # logging
                      print('{observer} not in list...'.format(observer))
  
          elif isinstance(observers, base.Observer):
              try:
                  index = self._observers.index(observers)
                  self._observers.remove(self._observers[index])
              except ValueError:
                  # logging
                  print('{observer} not in list...'.format(observers))
  
          else:
              err_message = ('ConfigReader.register support'
                             'ListType, TupleType and {observer} Object.'
                             ''.format(base.Observer.__name__)
                             )
  
              raise ValueError(err_message)