span class="special">{</span>
  <span class="comment">// types</span>
  <span class="keyword">typedef</span> <a class="link" href="tag/error_of.html" title="Struct template error_of">tag::error_of</a><span class="special">&lt;</span> <span class="keyword">typename</span> <a class="link" href="as_weighted_feature.html" title="Struct template as_weighted_feature">as_weighted_feature</a><span class="special">&lt;</span> <span class="identifier">Feature</span> <span class="special">&gt;</span><span class="speci