>ar</code> is
   empty, <code class="code"><span class="constructor">Some</span> x</code> (where <code class="code">x</code> is the value) if it is full.
   Raise <code class="code"><span class="constructor">Invalid_argument</span> <span class="string">"Weak.get"</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_cop