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Collections are a way for you to organize kata so that you can create your own training routines. Every collection you create is public and automatically sharable with other warriors. After you have added a few kata to a collection you and others can train on the kata contained within the collection.
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This kata was meant to be easy if only its wasn't so hard.
Not with rec((x + 1) % 9, y + 1), but with rec((x + 1) % 9, y). But even in this case, to move to the next row, the recursion would need to walk through the current row once again. And only in the second walk, when it is on the last cell, it checks that the cell is filled, it moves to the next row. If (y + 1) is used, then the recursion would change the row every time it meets empty cell.
Nice, readable solution