[Boost-users] astar help

I have a number of questions RE the astar-cities example. Why is the distance_heuristic class needed. Is it used to find the weight for a test vertex to the goal vertex? I put a breakpoint in distance_heuristic::operator() and it never gets called. Is it ever used? astar_search (... distance_heuristic ...) Do I have to define my own or supply an astar_heuristic? Or can I pass a default heuristic? astar_goal_visitor. Do I need to use this or can I pass a default default_astar_visitor? Are there other ways of reaching a goal without throwing an exception? Thanks. Any advice appreciated