= N &lt; I</code>. <code>IteratorType</code>
belongs to <code>const_iterator_type_list</code> or
<code>iterator_type_list</code>. <code>it</code> is a
valid (constant or non-constant) iterator of some index of <code>*this</code>
(i.e. does not refer to some other <code>multi_index_container</code>.)<br>
<b>Effects:</b> Returns an <code>nth_index&lt;N>::type::const_iterator</code>
equivalent to <code>it</code>.<br>
<b>Complexity:</b> Constant.<br>
<b>Exception safety:</b> <code>nothrow</code>.
</blockquote>
