er
      <small><em>Optional</em></small></h2>
    </div>
  </div>

  <div id="introduction">
    <p>This section of the tutorial is only for those who are interested
    in writing their own type-checkers. Others may skip this
    section.</p><!--Copied from the manual-->

    <p>Although the Checker Framework ships with <a href=
    "https://checkerframework.org/manual/#introduction">
    many checkers</a>, you may wish to write your own checker because
    there are other run-time problems you wish to pr