$vocabularies = variable_get('taxo_drill_down_vocabularies', array());

  $form['taxonomy'] = array(
    '#type' => 'select',
    '#multiple' => TRUE,
    '#size' => 10,
    '#title' => t('Categories'),
    '#tree' => TRUE,
  );
  foreach ($vocabularies as $v) {
    $form['taxonomy'][$v] = taxonomy_form($v);
  }
  
  $form['submit'] = array(
    '#type' => 'submit',
    '#value' => t('Search'),
  );

  return drupal_get_form('taxo_drill_down_page', $form);
}