hscale=auto;
a, b, c, d;
mark top;
a->c: Message 1;
c->d: Message 2;
mark mid1;
d->b: Message 3;
mark mid2;
b->c: Message 4;
c->a: Message 5;
vertical top->mid2 at a-: 3 msgs;
vertical mid1-> at d+: 3 msgs;
---: Further procedure may follow;
