## To Do: ##

Determine whether a puzzle has a unique solution in solveSudoku, and be able
to require uniqueness in generateSudoku.

playSudoku should allow the user to write "notes" on the cells.

Scores and ratings.

Make solveSudoku smarter, require less guessing.

Fix "Broken X11 window" warnings after playSudoku(display="tk") on Linux.
Fix Error in structure(...) after playSudoku(display="tk") on Windows.

Generalize to NxN, where N=9, 16, etc., and pass "map" around.

Replace print statement in solveSudoku with printSudoku().

Document color scheme, make it selectable (eg for colorblind people).

Make a "sudoku" class.