Hi! Thanks for posting this kata.

What are the rules for determining a valid sudoko? From reading the article, it could be that each digit from 1 to 9 should appear once in each of the 9 3x3 subgrids - or maybe each row and column should also contain each digit exactly 1 time, as suggested ny the sentence "For example, the same single integer may not appear twice in the same row, column, or any of the nine 3×3 subregions of the 9×9 playing board"? Maybe there are other constraints (why else starting the sentence with 'For example'?)

Please consider including the rules in the kata description.