[Boost-users] [BGL] property tag classes and "num"

From http://www.boost.org/libs/graph/doc/using_adjacency_list.html#sec:adjacency-...: | Custom Edge Properties | | Creating your own property types and properties is easy; just define a | tag class for your new property. The property tag class will need to | define num with a unique integer ID, and kind which should be either | edge_property_tag, vertex_property_tag, or graph_property_tag. | | struct flow_t { | typedef edge_property_tag kind; | }; | | struct capacity_t { | typedef edge_property_tag kind; | }; It works with structs like this. Am I missing something, or where is "num"?