code: func (n Base64BigInt) MarshalJSON() ([]byte, error) {
  	buf, err := ToBytes(Base64String(n))
  	if err != nil {
  		return nil, fmt.Errorf("could not decode base64: %v", err)
  	}
  	v := big.NewInt(0)
  	v.SetBytes(buf)
  	return []byte(v.String()), nil
  }
docs: // MarshalJSON marshals itself.