protected function validate_taxonomies($value) {
        $terms = explode(',', $value);

        foreach ($terms as $term) {
            if (!empty($term) && !array_key_exists($term, self::get_taxonomies_list())) {
                return new lang_string('invalidtaxonomy', 'core_competency', $term);
            }
        }

        return true;
    }