This function creates a checksum for the IBAN (International Bank Account Number) a. The IBAN must also contain the two digits that are meant for the checksum (characters 3 and 4), although the value itself is ignored. Spaces in the IBAN are also ignored. See examples.

If no valid IBAN is passed, the function generates an error.

The return value is the created checksum as a string (so that leading zeros are not lost). Please use the function on a field of type "String" for that reason.