r">Some</span> el)</code> sets the <code class="code">n</code>th cell of <code class="code">ar</code> to be a
   (full) pointer to <code class="code">el</code>; <code class="code"><span class="constructor">Weak</span>.set ar n <span class="constructor">None</span></code> 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</co