word">const</span> <span class="comment">//Observer</span>
   <span class="special">{</span>  <span class="keyword">return</span> <span class="identifier">value_</span><span class="special">;</span> <span class="special">}</span>
<span class="special">};</span>

<span class="keyword">namespace</span> <span class="identifier">boost</span><span class="special">{</span>

<span class="keyword">template</span><span class="special">&lt;&gt;</span>
<span class="keyword">struct</span> <span class="identifier">has_n