class Solver is working for entry level
Sudoku(String s) is working

--- exec-maven-plugin:1.2.1:exec (default-cli) @ sudoku1 ---
=== com.livehereandnow.sodoku.Main
=== Solver.toPrint() === start
**************
*  Question  *
**************
 --- Sudoku 9x9 ---
 5 3 . . 7 . . . .
 6 . . 1 9 5 . . .
 . 9 8 . . . . 6 .
 8 . . . 6 . . . 3
 4 . . 8 . 3 . . 1
 7 . . . 2 . . . 6
 . 6 . . . . 2 8 .
 . . . 4 1 9 . . 5
 . . . . 8 . . 7 9
known cells count: 30

**************
*   Answer   *
**************
 --- Sudoku 9x9 ---
 5 3 . . 7 . . . .
 6 . . 1 9 5 . . .
 1 9 8 . . . . 6 .
 8 . . . 6 . . . 3
 4 . . 8 . 3 . . 1
 7 . . . 2 . . . 6
 . 6 . . . . 2 8 .
 2 8 7 4 1 9 6 3 5
 . . . . 8 . . 7 9
known cells count: 36

**************
*  Possible  *
**************
--- Sudoku 9x9 possible --- (start)
row#1:{},{},{24},{26},{},{2468},{1489},{1249},{2489}
row#2:{},{247},{24},{},{},{},{3478},{24},{2478}
row#3:{},{},{},{23},{34},{24},{3457},{},{247}
row#4:{},{125},{1259},{579},{},{14},{4579},{2459},{}
row#5:{},{25},{2569},{},{57},{},{579},{259},{}
row#6:{},{15},{1359},{59},{},{14},{4589},{459},{}
row#7:{39},{},{13459},{357},{357},{},{},{},{49}
row#8:{},{},{},{},{},{},{},{},{}
row#9:{39},{145},{13459},{2356},{},{26},{149},{},{49}
possible value count:135

single possible value: 
--- Sudoku 9x9 possible --- (end)

=== Solver.toPrint() === end