code: func SubUint64(a uint64, b uint64) (uint64, error) {
  	if a < b {
  		return 0, ErrOverflow.GenWithStackByArgs("BIGINT UNSIGNED", fmt.Sprintf("(%d, %d)", a, b))
  	}
  	return a - b, nil
  }
docs: // SubUint64 subtracts uint64 a with b and returns uint64 if no overflow error.