 also five iterator helper class templates, each
    corresponding to a different iterator category. These classes cannot be
    used for <a href="#chaining">base class chaining</a>. The following
    summaries show that these class templates supply both the iterator
    operators from the <a href="#grpd_iter_oprs">iterator operator class
    templates</a> and the iterator typedef's required by the C++ standard
    (<code>iterator_category</code>, <code>value_type</code>,
    <i>etc.</i>).</p>

    <table c