ass="keyword">typename</span> <span class="identifier">tag</span><span class="special">&lt;</span><span class="identifier">Left</span><span class="special">&gt;</span><span class="special">::</span><span class="identifier">type</span><span class="special">,</span> 
         <span class="keyword">typename</span> RightTag <span class="special">=</span> <span class="keyword">typename</span> <span class="identifier">tag</span><span class="special">&lt;</span><span class="identifier">Right</span><span class="spe