ign">-&gt;</span>&nbsp;t<br>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<span class="keyword">val</span>&nbsp;singleton&nbsp;:&nbsp;elt&nbsp;<span class="keywordsign">-&gt;</span>&nbsp;t<br>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<span class="keyword">val</span>&nbsp;remove&nbsp;:&nbsp;elt&nbsp;<span class="keywordsign">-&gt;</span>&nbsp;t&nbsp;<span class="keywordsign">-&gt;</span>&nbsp;t<br>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<span class="keyword">val</span>&nbsp;union&nbsp;:&nbsp;t&nbsp;<span class="keywordsign">