code: func prefixNumberWithCountryCallingCode(
  	countryCallingCode int,
  	numberFormat PhoneNumberFormat,
  	formattedNumber *builder.Builder) {
  
  	// TODO(ttacon): add some sort of BulkWrite builder to builder.Builder
  	// also that name isn't too awesome...:)
  	newBuf := builder.NewBuilder(nil)
  	switch numberFormat {
  	case E164:
  		newBuf.WriteString(string(PLUS_SIGN))
  		newBuf.Write(strconv.AppendInt([]byte{}, int64(countryCallingCode), 10))
  		newBuf.Write(formattedNumber.Bytes())
  	case INTERNATIONAL:
  		newBuf.WriteString(string(PLUS_SIGN))
  		newBuf.Write(strconv.AppendInt([]byte{}, int64(countryCallingCode), 10))
  		newBuf.WriteString(" ")
  		newBuf.Write(formattedNumber.Bytes())
  	case RFC3966:
  		newBuf.WriteString(RFC3966_PREFIX)
  		newBuf.WriteString(string(PLUS_SIGN))
  		newBuf.Write(strconv.AppendInt([]byte{}, int64(countryCallingCode), 10))
  		newBuf.WriteString("-")
  		newBuf.Write(formattedNumber.Bytes())
  	case NATIONAL:
  		fallthrough
  	default:
  		newBuf.Write(formattedNumber.Bytes())
  	}
  	formattedNumber.ResetWith(newBuf.Bytes())
  }
docs: // A helper function that is used by format and formatByPattern.