From: http://www.call-with-current-continuation.org/microfleng/microfleng-1.tgz (examples/uxn/bounce.ghc)

bounce.ghc

% bouncing balls

-initialization(main).

main :- 
    poke(0x4c00, [0x3c, 0x7e, 0xff, 0xff, 0xff, 0xff, 0x7e, 0x3c], D1),
    set_colors(0x0ce9, 0x01c0, 0x0ce5, D2),
    start(D1, D2).

start([], []) :-
    set_screen_vector(S),
    foreach(ball(S), [[1, 1, 1, 2, 2],
                      [2, 504, 100, -3, -2],
                      [3, 1, 312, 3, 3],
                      [1, 100, 1, -3, 3],
                      [2, 100, 312, 3, -2]], _).

ball(S, [C, X, Y, Dx, Dy]) :-
    move(S, C, X, Y, X, Y, Dx, Dy).

move([_|S], C, X1, Y1, X, Y, Dx, Dy) :-
    draw_sprite(X1, Y1, 0x00, 0x4c00, D1),
    when(D1, draw_sprite(X, Y, C, 0x4c00, D2)),
    X2 is X + Dx,
    Y2 is Y + Dy,
    when(D2, advance(S, C, X, Y, X2, Y2, Dx, Dy)).
advance(S, C, X1, Y1, X, Y, Dx, Dy) :-
    X =< 0 |
    Dx2 is -Dx,
    advance(S, C, X1, Y1, 1, Y, Dx2, Dy).
advance(S, C, X1, Y1, X, Y, Dx, Dy) :-
    Y =< 0 |
    Dy2 is -Dy,
    advance(S, C, X1, Y1, X, 1, Dx, Dy2).
advance(S, C, X1, Y1, X, Y, Dx, Dy) :-
    X > 504 |
    Dx2 is -Dx,
    advance(S, C, X1, Y1, 504, Y, Dx2, Dy).
advance(S, C, X1, Y1, X, Y, Dx, Dy) :-
    Y > 312 |
    Dy2 is -Dy,
    advance(S, C, X1, Y1, X, 312, Dx, Dy2).
advance(S, C, X1, Y1, X, Y, Dx, Dy) :-
    otherwise |
    move(S, C, X1, Y1, X, Y, Dx, Dy).