old_contents: from six import iteritems
  unset = object()
  
  
  def of(d):
      return {k: v for k, v in iteritems(d) if v != unset}
  
  
new_contents: from six import iteritems
  unset = object()
  
  
  def of(d):
      """
      Remove unset values from a dict.
  
      :param dict d: A dict to strip.
      :return dict: A dict with unset values removed.
      return {k: v for k, v in iteritems(d) if v != unset}
  
subject: Document `values.py`