f an error occurs, <code class="code"><span class="constructor">Arg</span>.parse</code> exits the program, after printing
    to standard error an error message as follows:</p><ul>
<li>  The reason for the error: unknown option, invalid or missing argument, etc.</li>
<li>  <code class="code">usage_msg</code></li>
<li>  The list of options, each followed by the corresponding <code class="code">doc</code> string.
    Beware: options that have an empty <code class="code">doc</code> string will not be included 