STD For fsudoku very difficult (12 sec). From https://homepages.cwi.nl/~aeb/games/sudoku/solving26.html
 | . . . | . . . | 2 8 . |
 | . . . | . 9 5 | . . . |
 | . . . | . 4 . | . . . |

 | . . . | 8 . . | . 3 . |
 | . . 5 | . . 6 | . . . |
 | 8 . . | . . . | . . . |

 | 1 4 . | 7 . . | . . . |
 | . . . | . . . | 5 . 9 |
 | . 7 . | . . . | 6 . . |
