Anwendung: Graphen

knight n s = do
    a <- relation ((1,1),(n,n))
    m <- atmost s $ do
           i <- indices a ; return $ a ! i
    assert [m]
    sequence_ $ do
        p <- indices a
        return $ assert $ do
            q <- indices a
            guard $ p == q || reaches p q
            return $ a!q
    return $ decode a

reaches (px,py) (qx,qy) =
    5 == (px - qx)^2 + (py - qy)^2