_system</span> <span class="identifier">quantity</span><span class="special">&lt;</span><span class="identifier">Unit</span><span class="special">,</span><span class="identifier">Y</span><span class="special">&gt;</span> <span class="special">&gt;;</span>

<span class="keyword">struct</span> <span class="identifier">dimensionless_type</span><span class="special">;</span>
<span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">class</span> <span class="identifier">System</