How do we approach a counting exercise from Enumerative Combinatorics (Prof. Stanley's book)? I am baffled by the first question in the exercise of Chapter 3, Enumerative Combinatorics. I have no idea how to approach it. The question reads: > [3] What is the connection between a partially ordered set and itinerant salespersons who take revenge on customers who don’t pay their bills? Can somebody give me a reference? Thanks!

As Asaf suspects, it's a joke, and as Marc points out, there are solutions in the book. The edition I found on Google books has this on page 408:

Itinerant salespersons who take revenge on customers who don't pay their bills are retaliatory peddlers, and “retaliatory peddlers” is an anagram of “partially ordered set” (i.e., they have the same multiset of letters).