an><span class="special">::</span><span class="identifier">units</span><span class="special">::</span><span class="identifier">quantity</span><span class="special">&lt;</span> <span class="identifier">Unit2</span><span class="special">,</span> <span class="identifier">Y</span> <span class="special">&gt;</span><span class="special">&gt;</span> <span class="special">{</span>
  <span class="comment">// types</span>
  <span class="keyword">typedef</span> <a class="link" href="../units/multiply_typeof_helper.htm