mod_rest: Declare presence priority an integer in -128..127 per RFC6121 util.datamapper doesn't actually understand min/max, but it's informative and the schema could be used to validate the input at some point.

<tr>
  <th>What is {x} {op} {y}?</th><td>
    <input name="captcha_challenge" type="hidden" value="{challenge}">
    <input name="captcha_reply" pattern="[0-9]+" required type="number">
  </td></tr>