docs: def _with_primary(max_staleness, selection):
      """Apply max_staleness, in seconds, to a Selection with a known primary."""
      primary = selection.primary
      sds = []
  
      for s in selection.server_descriptions:
          if s.server_type == SERVER_TYPE.RSSecondary:
              # See max-staleness.rst for explanation of this formula.
              staleness = (
                  (s.last_update_time - s.last_write_date) -
                  (primary.last_update_time - primary.last_write_date) +
                  selection.heartbeat_frequency)
  
              if staleness <= max_staleness:
                  sds.append(s)
          else:
              sds.append(s)
  
      return selection.with_server_descriptions(sds)