docs: def short_string(self):
          """
          Compact string representation which doesn't print any of the
          collection elements.
          """
          source_str = ""
          if self.sources:
              source_str = " from '%s'" % ",".join(self.sources)
          return "<%s%s with %d elements>" % (
              self.__class__.__name__,
              source_str,
              len(self))