       the following invariant always holds:
          </p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc"><li class="listitem">
                <code class="computeroutput"><span class="identifier">traits</span><span class="special">::</span><span class="identifier">is_parser</span><span class="special">&lt;</span><span class="identifier">E</span><span class="special">&gt;::</span><span class="identifier">type</span></code> evaluates to <code class="computeroutput"><span class="identifier">