ypename</span> Time<span class="special">,</span> 
               <span class="keyword">typename</span> Observer<span class="special">&gt;</span> 
        <span class="identifier">boost</span><span class="special">::</span><span class="identifier">enable_if</span><span class="special">&lt;</span> <span class="keyword">typename</span> <span class="identifier">has_value_type</span><span class="special">&lt;</span> <span class="identifier">State</span> <span class="special">&gt;</span><span class="special">::<