s</span><span class="special">::</span><span class="identifier">unit</span><span class="special">&lt;</span> <span class="identifier">Dim2</span><span class="special">,</span> <span class="identifier">System2</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.html" title="Struct template multiply_typeof_helper">boost::units