anges to a property through a local vertex
    descriptor for one subgraph will change the property for the
    global vertex descriptor, and therefore for all other subgraphs.
    However, the key type for a subgraph's property map is a subgraph-local
    vertex or  edge descriptor. 
    (Required by <a href="PropertyGraph.html">PropertyGraph</a>.)

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<pre>
template &lt;class PropertyTag, class Key&gt;
typename property_traits&lt;
  typename property_map&lt;subgraph, PropertyTag&gt;::const_type
&gt;::va