def str2iso_reference(n):
cs_source = n + '<STR_LIT>'<EOL>cs = <NUM_LIT> - (iso_reference_str2int(cs_source) % <NUM_LIT>)<EOL>return "<STR_LIT>" % (cs, n)<EOL>
Calculates checksum (and adds the RF prefix) for an international reference number from a string (can contain any characters valid for the reference)