i1</i>(<i>v1</i>) != <i>i2</i>(<i>v2</i>), that candidate is rejected.
This mapping can be used either to speed up the search (as is done by the
default value, which requires that the degrees of <i>v1</i> and <i>v2</i> are
equal) or to impose extra conditions on the result.  The
<tt>VertexInvariant1</tt> and <tt>VertexInvariant2</tt> types must model <a
href="http://www.sgi.com/tech/stl/UnaryFunction.html">UnaryFunction</a>, with
the argument type of <tt>vertex_invariant1</tt> being <tt>Graph1</tt>'s vertex