OK, so: 1. I think I only got one grid that was unsolveable. 2. That was my first grid. 3. I was only playing with addition. 4. I completed exactly one roun

OK, so: 1. I think I only got one grid that was unsolveable. 2. That was my first grid. 3. I was only playing with addition. 4. I completed exactly one round. Because the "unsolveable" grid was my very first, I may well have pressed on without investigating too much, because perhaps things would become clearer; and indeed, all subsequent grids were easy to solve. So please don't treat my comment as a bug report! If you've taken trouble to avoid that outcome, then almost certainly the error is me.