docs: def get_identifiers(sysmeta_pyxb):
      """Get set of identifiers that provide revision context for SciObj.
  
      Returns:   tuple: PID, SID, OBSOLETES_PID, OBSOLETED_BY_PID
  
      """
      pid = d1_common.xml.get_opt_val(sysmeta_pyxb, 'identifier')
      sid = d1_common.xml.get_opt_val(sysmeta_pyxb, 'seriesId')
      obsoletes_pid = d1_common.xml.get_opt_val(sysmeta_pyxb, 'obsoletes')
      obsoleted_by_pid = d1_common.xml.get_opt_val(sysmeta_pyxb, 'obsoletedBy')
      return pid, sid, obsoletes_pid, obsoleted_by_pid