baseline"> A <a href="ref-glossary.html#id178">proper&nbsp;list</a>
</td></tr></table>
</p>
<p><span class="fndoclabel">Returns</span> <br />A list of <a href="ref-glossary.html#id74">extended&nbsp;unit-classes&nbsp;specifications</a>; <code>t</code>,
if instances of any <a href="ref-glossary.html#id254">unit&nbsp;class</a> are allowed on the
<a href="ref-glossary.html#id222">space&nbsp;instance</a>; or <code>nil</code>, if no <a href="ref-glossary.html#gl:instances">unit&nbsp;instances</a> are allowed on
t