cb1 :: Chessboard
cb1 = [[' ','B',' ',' ',' ','B','B',' '],
       ['B',' ',' ',' ',' ','B',' ',' '],
       [' ',' ',' ',' ',' ',' ',' ',' '],
       [' ',' ',' ',' ',' ',' ',' ',' '],
       [' ','B',' ',' ',' ',' ',' ',' '],
       [' ',' ',' ',' ',' ',' ','B',' '],
       [' ',' ',' ',' ',' ',' ',' ',' '],
       [' ',' ','B',' ',' ',' ',' ',' ']]

cb2 :: Chessboard
cb2 = [[' ','B',' ',' ',' ','B','B',' '],
       ['B',' ',' ',' ',' ','B',' ',' '],
       [' ',' ','c',' ',' ',' ',' ',' '],
       [' ',' ',' ',' ',' ',' ',' ','b'],
       [' ','B',' ',' ',' ',' ',' ',' '],
       [' ',' ',' ',' ','a',' ','B',' '],
       [' ',' ',' ',' ',' ',' ',' ',' '],
       [' ',' ','B',' ',' ',' ',' ',' ']]

cb3 :: Chessboard
cb3 = [[' ',' ','B',' ',' ',' ',' ',' '],
       [' ',' ',' ',' ','B',' ',' ',' '],
       [' ',' ',' ',' ',' ',' ','B',' '],
       ['B',' ',' ',' ',' ',' ',' ',' '],
       [' ',' ',' ','B',' ',' ',' ',' '],
       [' ','B',' ',' ',' ',' ',' ',' '],
       [' ',' ',' ',' ',' ',' ',' ','B'],
       [' ',' ',' ',' ',' ','B',' ',' ']]

cb4 :: Chessboard
cb4 = [[' ',' ',' ',' ',' ',' ',' ',' '],
       [' ',' ',' ',' ',' ',' ',' ',' '],
       [' ',' ',' ',' ',' ',' ',' ',' '],
       [' ',' ',' ',' ',' ',' ',' ',' '],
       [' ',' ',' ',' ',' ',' ',' ',' '],
       [' ',' ',' ',' ',' ',' ',' ',' '],
       [' ',' ',' ',' ',' ',' ',' ',' '],
       [' ',' ',' ',' ',' ',' ',' ',' ']]


isValidChessboard cb1 == True
isValidChessboard cb2 == False

threatens cb1 (1,1) == [(1,2),(2,1)]
threatens cb1 (0,5) == []

noThreats cb1 == False
noThreats cb2 == False

fullBoard [(1,1),(2,1)]  == cb4
fullBoard [(1,3),(6,2)]  == cb3

sdk1 :: Sudoku
sdk1 = [ [1,3,2,4],
         [2,0,3,0],
         [3,1,0,2],
         [0,2,0,3] ]

sdk2 :: Sudoku
sdk2 = [ [1,5,2,4],
         [2,0,3,0],
         [3,1,0,9],
         [0,2,0,3] ]

sdk3 :: Sudoku
sdk3 = [ [1,3,2,4],
         [2,4,3,1],
         [3,1,4,2],
         [4,2,1,3] ]

sdk4 :: Sudoku
sdk4 = [ [0,0,0,0],
         [0,0,0,0],
         [0,0,0,0],
         [0,0,0,0] ]

isValidStart 2 sdk1 == True
isValidStart 2 sdk2 == False

isValidStart 2 sdk3 == True
isValidStart 3 sdk3 == False

solve 2 sdk2 == sdk3 
solve 3 sdk2 == sdk4