Defining array_filter for settings checkboxes

Read the following comment, from system.module:

/**
 * Execute the system_settings_form.
 *
 * If you want node type configure style handling of your checkboxes,
 * add an array_filter value to your form
 *
 */
function system_settings_form_submit($form_id, $values) {...

What does this comment mean? Here's what I think it means. When you are writing the settings hook for a module that can be applied to various node types, you want checkboxes like

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But the settings don't save properly because of form problems with checkbox values. So the comment is instructing you to add an array_filter element to your form; it is picked up in the code ca. line 789:

      if (is_array($value) && isset($values['array_filter'])) {
        $value = array_keys(array_filter($value));
      }

After some trial and error, I got the saving of checkbox values for node types to work by defining the form element $form['array_filter'] = array('#type' => 'hidden'); (the #value key is not actually used; it's only checking if the key is set).

So to clarify this cryptic comment, I propose adding an example to the comment as follows:

/**
 * Execute the system_settings_form.
 *
 * If you want node type configure style handling of your checkboxes,
 * add an array_filter value to your form, e.g.,
 * $form['array_filter'] = array('#type' => 'hidden');
 *
 */
function system_settings_form_submit($form_id, $values) {...

chx, is this what the comment means? FYI, It was originally added here.