ph is a graph that has some property associated with each
of the vertices or edges in the graph. As a given graph may have
several properties associated with each vertex or edge, a tag is used
to identify which property is being accessed.  The graph provides a
function which returns a property map object.

<P>

<H3>Refinement of</H3>

<a href="./Graph.html">Graph</a>

<h3>Notation</h3>

<Table>
<TR>
<TD><tt>G</tt></TD>
<TD>A type that is a model of PropertyGraph.</TD>
</TR>

<TR>
<TD><tt>g</tt></TD>
<TD>An 