 class="keyword">operator</span><span class="special">(</span><span class="special">)</span><span class="special">(</span><span class="identifier">arg</span><span class="special">&lt;</span><span class="number">0</span><span class="special">&gt;</span><span class="special">::</span><span class="identifier">type</span> a1<span class="special">,</span> <span class="identifier">arg</span><span class="special">&lt;</span><span class="number">1</span><span class="special">&gt;</span><span class="special">::</spa