.  They are defined
in a namespace from which they can be accessed conveniently.
With the exception of <code class="literal">extent_gen</code> and 
<code class="literal">extent_range</code>, these types fulfill the roles of the
same name required by MultiArray and are described in its
concept definition.  <code class="literal">extent_gen</code> and
<code class="literal">extent_range</code> are described below.
</p>
</div>
<div class="sect2">
<div class="titlepage"><div><div><h3 class="title">
<a name="exten