sing is required, <em>end_symbol</em> may be a
    function that ultimately returns <code>1</code> (indicating a beginning fold point),
    <code>-1</code> (indicating an ending fold point), or <code>0</code> (indicating no fold point).
    That function is passed the following arguments:</p>

    <ul>
    <li><code>text</code>: The text being processed for fold points.</li>
    <li><code>pos</code>: The position in <em>text</em> of the beginning of the line currently
    being processed.</li>
    <li><code