ier">is_string</span></code> customization
          point is a template meta-function. It is used by <span class="emphasis"><em>Qi</em></span>
          <a class="link" href="../../qi/reference/string/string.html" title="String Parsers (string, lit)">String Literals</a>
          (<code class="computeroutput"><span class="identifier">lit</span><span class="special">(</span><span class="identifier">str</span><span class="special">)</span></code>),
          <span class="emphasis"><em>Qi</em></span> <a class