+-----+-----+-----+-----+-----+
            |3    |     |15   |     |13   |
            |     |     |     |     |     |
            |     |     |     |     |     |
+-----+-----+-----+-----+-----+-----+-----+-----+-----+
|16   |     |     |#####|     |#####|12   |     |5a   |
|     |     |     |#_#_#|     |#_#_#|     |     |     |
|     |     |     |#####|     |#####|     |     |     |
+-----+-----+-----+-----+-----+-----+-----+-----+-----+
|     |#####|20a  |5b   |     |20b  |     |#####|     |
|     |#_#_#|     |     |     |     |     |#_#_#|     | 1 2 6 0 9 6
|     |#####|     |     |     |     |     |#####|     |
+-----+-----+-----+-----+-----+-----+-----+-----+-----+
|8    |5c   |     |     |#####|19a  |     |17   |     |
|     |     |     |     |#_#_#|     |     |     |     |
|     |     |     |     |#####|     |     |     |     |
+-----+-----+-----+-----+-----+-----+-----+-----+-----+
|21   |     |#####|1    |     |     |#####|18   |     |
|     |     |#_#_#|     |     |     |#_#_#|     |     |
|     |     |#####|     |     |     |#####|     |     |
+-----+-----+-----+-----+-----+-----+-----+-----+-----+
      |5d   |     |     |     |19b  |     |     |
      |     |     |     |     |     |     |     |
      |     |     |     |     |     |     |     |
      +-----+-----+-----+     +-----+-----+-----+
                                 2     2     1
                                 4     9     2
                                       6     0
                                             9