span></summary><dl><dt class="spec type" id="type-key"><a href="#type-key" class="anchor"></a><code><span class="keyword">type</span> key</code></dt><dt class="spec type" id="type-t"><a href="#type-t" class="anchor"></a><code><span class="keyword">type</span> <span>+'a t</span></code></dt></dl><dl><dt class="spec value" id="val-empty"><a href="#val-empty" class="anchor"></a><code><span class="keyword">val</span> empty : <span><span class="type-var">'a</span> <a href="index.html#type-t">t</a></span></code></