##Vertical boxes
##vertical box
hscale=auto;
a, b, c, d;
{
    a->c: Message 1;
    c->d: Message 2;
    d->b: Message 3;
    b->c: Message 4;
    c->a: Message 5;
};
vertical box at a-:
       A process of\n5 messages;
---: Further procedure may follow;
