thout inducing memory leaks.
    Weak hash tables are defined on values from a <a href="Hashtbl.HashedType.html"><code class="code"><span class="constructor">Hashtbl</span>.<span class="constructor">HashedType</span></code></a>
    module; the <code class="code">equal</code> 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="