Classic sudoku puzzle

* endless (computer generated) puzzles in 4 difficulty levels
* many setting options for learning how to play perfectly
* (optionally) calculated digits (pencil marks)
* show errors/hints
* free mode: design your own puzzles or play a newspaper sudoku
* highscore

optionally:
* highlight errors
* highlight all occurrences of a selected number
* highlight finished numbers

digits (pencil marks):
* constantly compute all pencil marks as the sudoku changes
* calculate pencil marks at start
* show marks as numbers or as dots

Have fun playing!