n>&nbsp;+<span class="keywordsign">'</span>a&nbsp;t<br>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<span class="keyword">val</span>&nbsp;empty&nbsp;:&nbsp;<span class="keywordsign">'</span>a&nbsp;t<br>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<span class="keyword">val</span>&nbsp;is_empty&nbsp;:&nbsp;<span class="keywordsign">'</span>a&nbsp;t&nbsp;<span class="keywordsign">-&gt;</span>&nbsp;bool<br>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<span class="keyword">val</span>&nbsp;mem&nbsp;:&