SEPA creditor reference generator

The check digits follow ISO 7064 mod 97-10, the same routine an IBAN uses: the reference is rearranged with RF00 at the end, letters count as 10 to 35, and the check is 98 minus the mod 97 remainder. In validate mode a good reference leaves remainder 1 when RF and its digits move to the end. The standard's own worked example is RF18 5390 0754 7034, and the examples above include a deliberately broken twin so you can see both verdicts.