 sets the <code class="code">n</code>th
   cell of <code class="code">ar</code> to empty.
   Raise <code class="code"><span class="constructor">Invalid_argument</span> <span class="string">"Weak.set"</span></code> if <code class="code">n</code> is not in the range
   0 to <a href="Weak.html#VALlength"><code class="code"><span class="constructor">Weak</span>.length</code></a><code class="code"> a - 1</code>.<br>
</div>

<pre><span id="VALget"><span class="keyword">val</span> get</span> : <code class="type">'