        to do is to provide a reasonable observer. An example is
      </p>
<p>
</p>
<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">push_back_state_and_time</span>
<span class="special">{</span>
    <span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special">&lt;</span> <span class="identifier">state_type</span> <span class="special">&gt;&amp;</span> <span class="identifier">m_states</span><span class