docs: def listContents(self):
          """ Return list of volumes or diffs in this Store's selected directory. """
          items = list(self.extraKeys.items())
          items.sort(key=lambda t: t[1])
  
          (count, size) = (0, 0)
  
          for (diff, path) in items:
              if path.startswith("/"):
                  continue
              yield str(diff)
              count += 1
              size += diff.size
  
          yield "TOTAL: %d diffs %s" % (count, humanize(size))