 a range.</li>
<li><code class="code">r</code>: user-defined reader. Takes the next <code class="code">ri</code> formatted input
      function and applies it to the scanning buffer <code class="code">ib</code> to read the
      next argument. The input function <code class="code">ri</code> must therefore have type
      <code class="code"><span class="constructor">Scanning</span>.in_channel&nbsp;<span class="keywordsign">-&gt;</span>&nbsp;<span class="keywordsign">'</span>a</code> and the argument read has