<a href="#iterator">iterator
    helpers</a>, has an additional, but optional, template type parameter
    <code>B</code>. This parameter will be a publicly-derived base class of
    the instantiated template. This means it must be a class type. It can be
    used to avoid the bloating of object sizes that is commonly associated
    with multiple-inheritance from several empty base classes (see the <a
    href="#old_lib_note">note for users of older versions</a> for more
    details). To provide support for