is a function
<i>f</i> such that for all pairs of vertices <i>a,b</i> in
<i>V<sub>1</sub></i>, edge <i>(a,b)</i> is in <i>E<sub>1</sub></i> if
and only if edge <i>(f(a),f(b))</i> is in <i>E<sub>2</sub></i>.
</p>

<p>
This function returns <tt>true</tt> if there exists an isomorphism
between graph 1 and graph 2 and <tt>false</tt> otherwise. Also, if a
isomorphism map named parameter is provided then an isomorphism is
recorded in the map.
</p>

<p>
The current implementation is based on descriptions of a back