class="keyword">struct</span> <span class="identifier">dimensionless_quantity</span><span class="special">&lt;</span><span class="identifier">System</span><span class="special">,</span><span class="identifier">Y</span><span class="special">&gt;;</span>

<span class="keyword">struct</span> <span class="identifier">implicitly_convertible</span><span class="special">;</span>
<span class="keyword">struct</span> <span class="identifier">trivial_conversion</span><span class="special">;</span>
<span class="keyword