tt>true</tt> if the graph is the root of the subgraph tree,
    and returns <tt>false</tt> otherwise.

<hr>
<pre>
subgraph& parent()
</pre>
    Returns the parent graph.

<hr>
<pre>
std::pair&lt;children_iterator, children_iterator&gt; children() const
</pre>
Return an iterator pair for accessing the children subgraphs. 


<!----------------------------->
<h3>Nonmember Functions</h3>

The functionality of <tt>subgraph</tt> depends on the
<tt>Graph</tt> type. For example, if <tt>Graph</tt> in a
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