   or because keys are not hashable/comparable with the polymorphic builtins.
<p>

    For instance, one might want to specialize a table for integer keys:
    </p><pre class="codepre"><code class="code">      <span class="keyword">module</span> <span class="constructor">IntHash</span> =
        <span class="keyword">struct</span>
          <span class="keyword">type</span> t = int
          <span class="keyword">let</span> equal i j = i=j
          <span class="keyword">let</span> hash i = i <span class="k