It's murder in Discworld! -- which ordinarily is no big deal. But what bothers Watch Commander Sir Sam Vimes is that the unusual deaths of three elderly Ankh-Morporkians do not bear the clean, efficient marks of the Assassins' Guild. An apparent lack of any motive is also quite troubling. All Vimes has are some tracks of white clay and more of those bothersome "clue" things that only serve to muck up an investigation. The anger…