code: func exportSet(bits int64, on, off, separator string, numberOfBits int64) string {
  	result := ""
  	for i := uint64(0); i < uint64(numberOfBits); i++ {
  		if (bits & (1 << i)) > 0 {
  			result += on
  		} else {
  			result += off
  		}
  		if i < uint64(numberOfBits)-1 {
  			result += separator
  		}
  	}
  	return result
  }
docs: // exportSet evals EXPORT_SET(bits,on,off,separator,number_of_bits).
  // See https://dev.mysql.com/doc/refman/5.7/en/string-functions.html#function_export-set