a, b, c, d;
a->b: Message 1
      goes here;
mark m1;
b->c: Message 2
      goes here;
mark m2;
c->d: Messgae 3
      goes here;
defstyle symbol [fill.color="yellow,200"];
symbol rectangle m1-m2 left at a+ [xsize=30];
symbol rectangle   -m2 left at b+ [xsize=30, ysize=30];
symbol rectangle m1-   left at c+ [xsize=30, ysize=30];