lass="identifier">unique</span><span class="special">&amp;);</span>

        <span class="comment">// dereference is called whenever multi_pass::operator*() is invoked</span>
        <span class="comment">//</span>
        <span class="comment">//   mp:    is a reference to the whole multi_pass instance</span>
        <span class="comment">//</span>
        <span class="comment">// This function is expected to return a reference to the current</span>
        <span class="comment">// character/token.</span>
