 <span class="keyword">class</span> <span class="identifier">R</span><span class="special">,</span> <span class="keyword">class</span> <span class="identifier">D</span>
<span class="special">&gt;</span>
<span class="keyword">typename</span> <span class="identifier">iterator_adaptor</span><span class="special">::</span><span class="identifier">difference_type</span> <span class="identifier">distance_to</span><span class="special">(</span>
    <span class="identifier">iterator_adaptor</span><span class="speci