de class="code"><span class="constructor">Array</span>.set a n x</code> modifies array <code class="code">a</code> in place, replacing
   element number <code class="code">n</code> with <code class="code">x</code>.
   You can also write <code class="code">a.(n) &lt;- x</code> instead of <code class="code"><span class="constructor">Array</span>.set a n x</code>.
<p>

   Raise <code class="code"><span class="constructor">Invalid_argument</span> <span class="string">"index out of bounds"</span></code>
   if <c