     is a type used as an attribute customization point. It is invoked by the
          <span class="emphasis"><em>Qi</em></span> repetitive parsers (<a class="link" href="../../qi/reference/operator/kleene.html" title="Kleene Parser (*a)">Kleene</a>,
          <a class="link" href="../../qi/reference/operator/plus.html" title="Plus Parser (+a)">Plus</a>, <a class="link" href="../../qi/reference/operator/list.html" title="List Parser (a % b)">List</a>,
          and <a class="link" href="../../qi/reference/