de class="code">a</code>.</p>

<p>Raise <code class="code"><span class="constructor">Invalid_argument</span>&nbsp;<span class="string">"Array.sub"</span></code> if <code class="code">start</code> and <code class="code">len</code> do not
   designate a valid subarray of <code class="code">a</code>; that is, if
   <code class="code">start&nbsp;&lt;&nbsp;0</code>, or <code class="code">len&nbsp;&lt;&nbsp;0</code>, or <code class="code">start&nbsp;+&nbsp;len&nbsp;&gt;&nbsp;<span class="constructor">Array</span>