</pre>
    If vertex <i>u</i> is in this subgraph, the function returns the local
    vertex descriptor that corresponds to the global vertex descriptor
    <tt>u_global</tt> as the first part of the pair and <tt>true</tt> for
    the second part of the pair. If vertex <i>u</i> is not in the subgraph
    then this function returns false in the second part of the
    pair.

<hr>
<pre>
subgraph& root()
</pre>
    Returns the root graph of the subgraph tree.

<hr>
<pre>
bool is_root() const
</pre>
    Return <