">template</span><span class="special">&lt;</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">,</span><span class="keyword">class</span> <span class="identifier">S1</span><span class="special">,</span><span class="keyword">class</span> <span class="identifier">S2</span><span class="special">&gt;</span> <span class="keyword">struct</span> <span class="identifier">base_unit_converter</span><span class="special">&lt;</span><span class="identifier">T</span><span cla