Keys "s"=save "l"=load "b"=back once
After you move, you designate an empty square that the opponent may not occupy. The program requires you to do this. The same player may not designate the same square on three successive turns. D. B. Pritchard, 1989.
There is another version in which the barred square may not be crossed. I do not know whether it is lawful to escape check or mate simply by barring a square.
These are simple illustrations rather than strong opponents.
Bug reports? -- Thank you! Keep them coming!
