pe-erases the callback without requiring a dynamic allocation.
                  For this reason, the callback object is passed by a non-constant
                  reference.
                </p>
                <p>
                  The function object will be called with this equivalent signature:
                </p>
<pre class="programlisting"><span class="keyword">void</span>
<span class="identifier">callback</span><span class="special">(</span>
    <span class="identifier">std</span><span class="speci