y a non-constant reference.
                </p>
                <p>
                  The function object will be called with this equivalent signature:
                </p>
<pre class="programlisting"><span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span>
<span class="identifier">callback</span><span class="special">(</span>
    <span class="identifier">std</span><span class="special">::</span><span class="identifier">uint64_t</span> <span class="identifie