er">i</span>  <span class="special">|</span>    <span class="identifier">i</span>        <span class="identifier">S1</span> <span class="identifier">S2</span> <span class="identifier">S3</span>      
                                   <span class="identifier">s</span>  <span class="special">|</span> <span class="identifier">s</span>   <span class="identifier">s</span>              <span class="identifier">b</span>  <span class="special">|</span>                      <span class="identifier">M1</span> <span 