a, b, c, d;
vspace 0;
activate a, b, c;
a->b: Message 1
      goes here;
mark m1;
b->c: Message 2
      goes here;
mark m2;
c->d: Messgae 3
      goes here;
deactivate a, b, c;
---: end;
defstyle symbol [fill.color="yellow,200"];
symbol rectangle m1-m2 center at a- [xsize=30, draw_time=before_entity_lines];
symbol rectangle   -m2 center at b  [xsize=30, ysize=30, draw_time=after_entity_lines];
symbol rectangle m1-   center at c- [xsize=30, ysize=30, draw_time=after_default];

