fo</span>
    <span class="comment">//</span>
    <span class="comment">//  p:          a parser</span>
    <span class="comment">//  context:    enclosing rule context (can be unused_type)</span>

    <span class="comment">// Requirement: P::template attribute&lt;Ctx, Iter&gt;::type</span>
    <span class="comment">//</span>
    <span class="comment">//  P:          a parser type</span>
    <span class="comment">//  Ctx:        A context type (can be unused_type)</span>
    <span class="comment">//  Iter: 