r.watershed uses the A* search algorithm to find the flow net. The MAN
page has a reference to a conference proceeding's paper on how the A* search
algorithm is used in r.watershed. The A* algorithm is well documented in
artificial intellegence publications in the early and mid '80s.
The diference between r.watershed and r.watershed4.0 is that r.watershed (for
4.1) has a bug in it. r.watershed4.0 doesn't.
Chuck Ehlschlaeger