as <i>(E,B)</i> and <i>(F,D)</i> that cross
out of a subgraph are not in the edge set of the subgraph.
</p>

<P></P>
<DIV ALIGN="center"><A NAME="fig:subgraph-tree"></A>
<TABLE>
<CAPTION ALIGN="BOTTOM"><STRONG>Figure 1:</STRONG> A graph with nested subgraphs, maintained in a tree structure.</CAPTION>
<TR><TD><IMG SRC="./figs/subgraph.gif"></TD>
<TD><IMG SRC="./figs/subgraph-tree.gif"></TD></TR>
</TABLE>
</DIV><P></P>

<p>The <tt>subgraph</tt> class implements induced subgraphs. The main graph
and its subgra