code: def _compound_column_value(k1, k2):
          
          k1 = ProjectStats._column_value(k1)
          k2 = ProjectStats._column_value(k2)
          if k1 == 'unknown' and k2 == 'unknown':
              return 'unknown'
          return '%s %s' % (k1, k2)
docs: Like :py:meth:`~._column_value` but collapses two unknowns into one.
  
          :param k1: first (top-level) value
          :param k2: second (bottom-level) value
          :return: display key
          :rtype: str