The core of this program lives in the solver.py file, the questioner.py, and the grid.py file. The grid.py has a class that wraps around a 9x9x9 array representing possible values for a grid. The Sudoku Grid extends this class to provide some other abilities. In the solver.py file, the program determines each cell one at a time (and checkpoints at the right time). The questioner.py file routes questions to the user or plays itself accordingly.