[Boost-users] Newbie question: add_edge()

Thanks for all the help so far. I'm not there yet, though! The documentation for add_edge() says that the first two parameters are of type boost::graph_traits<G>::vertex_descriptor. In the example in quick_tour.html it is called with integers (from an enum) as its first two parameters. Is that because: i. add_edge is overloaded to take integers as the first two parameters; ii. boost::graph_traits<G>::vertex_descriptor just happens to be an integer type in this case; iii. boost::graph_traits<G>::vertex_descriptor is /always/ an integer type; or iv. some other reason I haven't thought of. Thanks in advance.