(@ brightness:r color +-                                             brightness -r = color ->
    (" color.                                                          color.
        (# round ((/ 3) (+ red                                           round ((red +
                           green                                                 green +
                           blue)))                                               blue) / 3)
    )                                                                  ;
+                                                                    _
    {rgb} color                                                        color : rgb
)                                                                    ;


