map object for an internally stored property.
<p>The BGL <tt>adjacency_list</tt> class allows users to specify internally
stored properties through plug-in template parameters of the graph class. How to
do this is discussed in detail in Section <a href="using_adjacency_list.html#sec:adjacency-list-properties">Internal
Properties</a>. Externally stored properties can be created in many different
ways, although they are ultimately passed as separate arguments to the graph
algorithms. One straightforward way t