ode">s</code> via its index in the sequence. Indexes start at
   <code class="code">0</code>, and we will call an index valid in <code class="code">s</code> if it falls within the
   range <code class="code">[0...l-1]</code> (inclusive). A position is the point between two
   bytes or at the beginning or end of the sequence.  We call a
   position valid in <code class="code">s</code> if it falls within the range <code class="code">[0...l]</code>
   (inclusive). Note that the byte at index <code class="code"