Options form subform for exposed filter options.
Overrides FilterPluginBase::buildExposeForm
buildOptionsForm()
public function buildExposeForm(&$form, &$form_state) {
parent::buildExposeForm($form, $form_state);
$form['expose']['reduce'] = array(
'#type' => 'checkbox',
'#title' => t('Limit list to selected items'),
'#description' => t('If checked, the only items presented to the user will be the ones selected here.'),
'#default_value' => !empty($this->options['expose']['reduce']),
);
}