Ajax form callback: Selects the 'drivernumber' element of the validation form.
function ajax_forms_test_validation_number_form_callback($form, $form_state) {
drupal_set_message("ajax_forms_test_validation_number_form_callback invoked");
drupal_set_message(t("Callback: drivernumber=%drivernumber, spare_required_field=%spare_required_field", array(
'%drivernumber' => $form_state['values']['drivernumber'],
'%spare_required_field' => $form_state['values']['spare_required_field'],
)));
return '<div id="message_area_number">ajax_forms_test_validation_number_form_callback at ' . date('c') . '</div>';
}