   Class <code class="computeroutput"><span class="identifier">number</span></code> is expression-template-enabled:
      that means that rather than having a multiplication operator that looks like
      this:
    </p>
<pre class="programlisting"><span class="keyword">template</span> <span class="special">&lt;</span><span class="keyword">class</span> <span class="identifier">Backend</span><span class="special">&gt;</span>
<span class="identifier">number</span><span class="special">&lt;</span><span class="i