ring. The <code class="code">i</code>th byte index is between position
       <code class="code">i</code> and <code class="code">i+1</code>.</li>
</ul>

<p>Two integers <code class="code">start</code> and <code class="code">len</code> are said to define a <em>valid
    substring</em> of <code class="code">s</code> if <code class="code">len&nbsp;&gt;=&nbsp;0</code> and <code class="code">start</code>, <code class="code">start+len</code> are
    positions of <code class="code">s</code>.</p>

<p><b>Unicode tex