 class="info">
<code class="code"><span class="constructor">Arg</span>.usage speclist usage_msg</code> prints to standard error
    an error message that includes the list of valid options.  This is
    the same message that <a href="Arg.html#VALparse"><code class="code"><span class="constructor">Arg</span>.parse</code></a> prints in case of error.
    <code class="code">speclist</code> and <code class="code">usage_msg</code> are the same as for <code class="code"><span class="constructor">Arg</span>.parse<