Note my attempt to use headport and tailport; these directives
are ignored. That's not completely a surprise, based on the
comment in Section 3.1 of "Drawing Graphs with Dot". But after
many attempts, I've not been able to come up with anything
better than what I've attached. Most attempts are far worse,
in that they superimpose both the arrows and the labels.
Input file: b531.dot
Output file: b531.pdf