open Builder for subtypes

[bastie]

Hello,
nice project.

Please do not Iban.Builder as final and declare fillMissingFieldsRandomly as protected, because generation of random Iban are rare correct with more checks. For example:

Germanys IBAN checksum maybe correct, but in 100_000 random cycles generates less than 10 times "more" correct IBAN. German IBAN contains the old Bankleitzahl after the checksum. The Bundesbank provides 4 times a year a list of valid Bankleitzahl. And so today's random generate IBAN for CountryCode DE is rare valid.
The Bankleitzahl is not the only check for German IBAN, because after that number came the Kontonummer and the Kontonummer include also a checksum depending of Bankleitzahl before.
See https://www.bundesbank.de/de/aufgaben/unbarer-zahlungsverkehr/serviceangebot/iban-regeln

If you open the Iban.Builder more accurate random build can be direct implemented.

with regards

[hajk1]

Hello @bastie, thanks for your comment. I'll consider your points.