a, b;
--: Overall [fill.color = blue+50,
             fill.gradient=up] {
    a--b [fill.color=red]:     solid;
    a--b [fill.color=red,128]: transparent;
    a--b [fill.color=red-50]:  dark red;
    a--b [fill.color=red+50]:  light red;
    a--b [fill.color=none]:    none;
    defstyle arrow [line.color=red, 
                    arrow.color=green, 
                    text.color=blue];
    a->b: Unmodified;
    a->b: Yellow overcast [color=++yellow,150];
};

