9 is an edge case that is very important to keep testing in the advanced Luhn iteration exercises (Luhn From and Luhn Trait).
I know it because I was optimizing my solutions and after passing all the From and Trait tests, when I put those optimizations in my original Luhn solution, it failed the Luhn tests. Turns out the canonical specification had a test case for this! So I’m adding it to the advanced exercises as well.