class=identifier>nth_index</span>                <span class=special>{</span><span class=keyword>typedef</span> <b>implementation defined</b> <span class=identifier>type</span><span class=special>;};</span>
  <span class=keyword>template</span><span class=special>&lt;</span><span class=keyword>typename</span> <span class=identifier>Tag</span><span class=special>&gt;</span>
  <span class=keyword>struct</span> <span class=identifier>index</span>                    <span class=special>{</span><span class=keywo