During the works on HV-809 it turned out that apparently two different check digit algorithms exist which are referred to as "Mod 10" check:

• Luhn algorithm
• Other Mod10 checks such as [ISBN-13|http://www.activebarcode.com/codes/checkdigit/modulo10.html|

The difference is how products calculated for the individual numbers handled. In the Luhn algorithm, 9 is subtracted when such a product is larger than 9, in the other algorithms these products are taken as is.

We may add another option to @Mod10 allowing to configure this behavior.