%     1   2   3   4   5   6   7   8
% 1 |   |   |   |   |   |   |   | x |
% 2 | x |   |   |   |   |   | x |   |
% 3 |   | x |   |   |   | x |   |   |
% 4 |   |   | x |   | x |   |	|   |
% 5 |   |   |   | B |   |   |   |   |
% 6 |   |   | x |   | x |   |   |   |
% 7 |   | x |   |   |   | x |   |   |
% 8 | x |   |   |   |   |   | x |   |

bishop(X1,Y1,X2,Y2) :- ((X2 is X1+1,Y2 is Y1-1); % right, up
		        (X2 is X1+1,Y2 is Y1+1); % right, down
		        (X2 is X1-1,Y2 is Y1-1); % left,  up
		        (X2 is X1-1,Y2 is Y1+1)),% left,  down
	                1=<X2,X2=<3,
	                1=<Y2,Y2=<3.

move_user(X1,Y1,X2,Y2) :- retractall(visited(ZX,ZY)),
	                  asserta(visited(X1,Y1)),
	                  asserta(visited(X2,Y2)),
	                  move(X1,Y1,X2,Y2).

move(X1,Y1,X2,Y2) :- bishop(X1,Y1,X2,Y2).
move(X1,Y1,X2,Y2) :- bishop(X1,Y1,ZX,ZY),
	             not(visited(ZX,ZY)),
	             asserta(visited(ZX,ZY)),
	             print(ZX),print(','),print(ZY),print('='),
		     N is (ZY-1)*3+ZX,print(N),nl,
	             move(ZX,ZY,X2,Y2).