vecS</TT> uses integers for vertex descriptors (indexed
from zero to the number of vertices in the graph), so they are
suitable as the key type for a pointer property map. When the
<TT>VertexList</TT> is not <TT>vecS</TT>, then the vertex descriptor is
not an integer, and cannot be used with a pointer property map.
Instead the method described above of using a
<TT>random_access_iterator_property_map</TT> with an ID
property must be used. The <TT>edge_list</TT> class may also use an
integer type for the vert