protected function add_collection_selector($mform) {
        $tagcolls = core_tag_collection::get_collections_menu(false, false, get_string('anycollection', 'block_tags'));
        if (count($tagcolls) <= 1) {
            return;
        }

        $tagcollssearchable = core_tag_collection::get_collections_menu(false, true);
        $hasunsearchable = false;
        foreach ($tagcolls as $id => $name) {
            if ($id && !array_key_exists($id, $tagcollssearchable)) {
                $hasunsearchable = true;
                $tagcolls[$id] = $name . '*';
            }
        }

        $mform->addElement('select', 'config_tagcoll', get_string('tagcollection', 'block_tags'), $tagcolls);
        if ($hasunsearchable) {
            $mform->addHelpButton('config_tagcoll', 'tagcollection', 'block_tags');
        }
        $mform->setDefault('config_tagcoll', 0);
    }