*
* TODO:
*
- * - can we do anything about nasty centring of text in GTK? It
- * seems to be taking ascenders/descenders into account when
- * centring. Ick.
- *
* - it might still be nice to do some prioritisation on the
* removal of numbers from the grid
* + one possibility is to try to minimise the maximum number
* of filled squares in any block, which in particular ought
* to enforce never leaving a completely filled block in the
* puzzle as presented.
- * + be careful of being too clever here, though, until after
- * I've tried implementing difficulty levels. It's not
- * impossible that those might impose much more important
- * constraints on this process.
*
* - alternative interface modes
* + sudoku.com's Windows program has a palette of possible