old_contents: """A set of Resource objects
  
  """
  
  class ResourceSet(dict):
      """Default implementation of class to store resources in ResourceList
  
      Key properties of this class are:
      - has add(resource) method
      - is iterable and results given in alphanumeric order by resource.uri
      """
  
      def __iter__(self):
          """Iterator over all the resources in this resource_list"""
          self._iter_next_list = sorted(self.keys())
          self._iter_next_list.reverse()
          return(iter(self._iter_next, None))
  
      def _iter_next(self):
          if (len(self._iter_next_list)>0):
              return(self[self._iter_next_list.pop()])
          else:
              return(None)
  
      def add(self, resource, replace=False):
          """Add just a single resource"""
          uri = resource.uri
          if (uri in self and not replace):
              raise ResourceSetDupeError("Attempt to add resource already in this set") 
          self[uri]=resource
  
  class ResourceSetDupeError(Exception):
      pass
  
new_contents: """A set of Resource objects used for Capability List
  Indexes and ResourceSync Description documents.
  
  FIXME - what should the ordering be?
  """
  
  class ResourceSet(dict):
      """Implementation of class to store resources in Capability List
      Indexes and ResourceSync Description documents.
  
      Key properties of this class are:
      - has add(resource) method
      - is iterable and results given in alphanumeric order by resource.uri
      """
  
      def __iter__(self):
          """Iterator over all the resources in this resource_list"""
          self._iter_next_list = sorted(self.keys())
          self._iter_next_list.reverse()
          return(iter(self._iter_next, None))
  
      def _iter_next(self):
          if (len(self._iter_next_list)>0):
              return(self[self._iter_next_list.pop()])
          else:
              return(None)
  
      def add(self, resource, replace=False):
          """Add just a single resource"""
          uri = resource.uri
          if (uri in self and not replace):
              raise ResourceSetDupeError("Attempt to add resource already in this set") 
          self[uri]=resource
  
  class ResourceSetDupeError(Exception):
      pass
  
subject: Improve comments