code: func (sm *SegmentMap) Set(key []byte, value interface{}, force bool) bool {
  	idx := int64(crc32.Checksum(key, sm.crcTable)) % sm.size
  	k := string(key)
  	_, exist := sm.maps[idx][k]
  	if exist && !force {
  		return exist
  	}
  
  	sm.maps[idx][k] = value
  	return exist
  }
docs: // Set if key not exists, returns whether already exists.