

<blockquote>
<b>Requires:</b> <code>Tag</code> is such that <code>index&lt;Tag>::type</code>
is valid.<br>
<b>Effects:</b> Returns a <code>const</code> reference to the 
<code>index&lt;Tag>::type</code> index held by
<code>*this</code>.<br>
<b>Complexity:</b> Constant.<br>
</blockquote>

<h4><a name="projection">Projection operations</a></h4>

<p>
Given a <code>multi_index_container</code> with indices <code>i1</code>
and <code>i2</code>, we say than an <code>i1</code>-iterator
<code>it1</code> and an <co