from talk with Steven North). -->

<p>
The subgraph class provides a mechanism for keeping track of a graph
and its subgraphs. A graph <i>G'</i> is a <i>subgraph</i> of a graph
<i>G</i> if the vertex set of <i>G'</i> is a subset of the vertex set
of <i>G</i> and if the edge set of <i>G'</i> is a subset of the edge
set of <i>G</i>. That is, if <i>G'=(V',E')</i> and <i>G=(V,E)</i>,
then <i>G'</i> is a subgraph of <i>G</i> if <i>V'</i> is a subset of
<i>V</i> and <i>E</i> is a subset of <i>E'</i>. An <i>induce