, attr) -&gt; bool</span>
    <span class="comment">//</span>
    <span class="comment">//  p:          a parser</span>
    <span class="comment">//  f, l:       first/last iterator pair</span>
    <span class="comment">//  context:    enclosing rule context (can be unused_type)</span>
    <span class="comment">//  skip:       skipper (can be unused_type)</span>
    <span class="comment">//  attr:       attribute (can be unused_type)</span>

    <span class="comment">// Requirement: p.what(context) -&gt; in