ode> relation and <code class="code">hash</code> function are taken from that
    module.  We will say that <code class="code">v</code> is an instance of <code class="code">x</code> if <code class="code">equal x v</code>
    is <code class="code"><span class="keyword">true</span></code>.
<p>

    The <code class="code">equal</code> relation must be able to work on a shallow copy of
    the values and give the same result as with the values themselves.<br>

</p><pre><span class="keyword">module type</span> <