 <tr>
      <th>callback</th>
      <td><p>a callback to be applied to this tool - either a character string of javascript code or any one of <code><a href='debug_callback.html'>debug_callback</a></code>, <code><a href='shiny_callback.html'>shiny_callback</a></code>, <code><a href='console_callback.html'>console_callback</a></code>, <code><a href='custom_callback.html'>custom_callback</a></code></p></td>
    </tr>
    <tr>
      <th>ref_layer</th>
      <td><p>name of the layer that the callback should be a