(VAR x y z )
(RULES 
        qsort(nil) -> nil
        qsort(.(x, y)) -> ++(qsort(lowers(x, y)), .(x, qsort(greaters(x, y))))
        lowers(x, nil) -> nil
        lowers(x, .(y, z)) -> if(<=(y, x), .(y, lowers(x, z)), lowers(x, z))
        greaters(x, nil) -> nil
        greaters(x, .(y, z)) -> if(<=(y, x), greaters(x, z), .(y, greaters(x, z)))
        
)