mark top;
source ,middle1, middle2, destination;
vspace 10;
source->destination: \plmessage \#1;
source->destination: \plmessage \#2;
mark a_top;
source->destination: \plmessage \#3;
mark a_bottom [offset=10];
symbol ... center at source-middle1;
source->destination: \plmessage \#n;
mark bottom;
symbol rectangle top-bottom left at middle1 -40 right at middle2 +40
    [fill.color=lgray, line.type=none,
    draw_time=before_entity_lines];
symbol arc a_top-a_bottom center at destination
    [xsize=60, line.color=red, line.width=3,
    fill.color=none];
