 it comes essentially "for
        free", in which case we might as well make use of it and use Halley-iteration.
        This is quite a typical situation when inverting special-functions. Here's
        the revised functor:
      </p>
<pre class="programlisting"><span class="keyword">template</span> <span class="special">&lt;</span><span class="keyword">class</span> <span class="identifier">T</span> <span class="special">=</span> <span class="keyword">double</span><span class="special">&gt;</span>
<span c