Simple chessboard that doesn't restrict piece movement.
The entire application consists of one screen, this screen consists of
a chessboard, initially in the starting position of traditional chess,
extra pieces below the board, and two buttons labeled 'undo' and 'redo'.
To move a piece, touch it, and then touch the square you wish to move it to.
To move a piece off the board, touch it and then touch an area outside the board.
To move one of the extra pieces onto the board, touch one of the extra pieces outside
the board and then touch the square you wish to move it to.
If you place a piece on a square where a different piece is placed, the former inhabitant is deleted.
If you press the undo button, the last movement is undone.
If you press the redo button, the last undoing is undone.