>n</code> is between positions
   <code class="code">n</code> and <code class="code">n+1</code>.</p>

<p>Two parameters <code class="code">start</code> and <code class="code">len</code> are said to designate a valid
   range of <code class="code">s</code> if <code class="code">len&nbsp;&gt;=&nbsp;0</code> and <code class="code">start</code> and <code class="code">start+len</code> are valid
   positions in <code class="code">s</code>.</p>

<p>Byte sequences can be modified in place, for instance via the <cod