 index held by the <code>multi_index_container</code>, in
the same order as they were specified.<br>
<b>Requires:</b> <code>0 &lt;= N &lt; I</code>.
</blockquote>

<code>template&lt;typename Tag> struct index</code>

<blockquote>
<code>index&lt;Tag>::type</code> yields the type of the index which
has <code>Tag</code> as an associated <a href="indices.html#tags">tag type</a>.<br>
<b>Requires:</b> Some index of the <code>multi_index_container</code> has <code>Tag</code>
as an associated tag type.
</blockquote