identifier">Dim</span><span class="special">,</span><span class="keyword">class</span> <span class="identifier">System</span><span class="special">,</span><span class="keyword">class</span> <span class="identifier">Y</span><span class="special">&gt;</span> <span class="keyword">struct</span> <span class="identifier">is_quantity_of_dimension</span><span class="special">&lt;</span> <span class="identifier">quantity</span><span class="special">&lt;</span><span class="identifier">unit</span><span class="special