public function expandIdentifiers(&$attr, $attr_types)
    {
        // because foreach will process new elements we add, make sure we
        // skip duplicates
        $processed = array();

        foreach ($attr as $def_i => $def) {
            // skip inclusions
            if ($def_i === 0) {
                continue;
            }

            if (isset($processed[$def_i])) {
                continue;
            }

            // determine whether or not attribute is required
            if ($required = (strpos($def_i, '*') !== false)) {
                // rename the definition
                unset($attr[$def_i]);
                $def_i = trim($def_i, '*');
                $attr[$def_i] = $def;
            }

            $processed[$def_i] = true;

            // if we've already got a literal object, move on
            if (is_object($def)) {
                // preserve previous required
                $attr[$def_i]->required = ($required || $attr[$def_i]->required);
                continue;
            }

            if ($def === false) {
                unset($attr[$def_i]);
                continue;
            }

            if ($t = $attr_types->get($def)) {
                $attr[$def_i] = $t;
                $attr[$def_i]->required = $required;
            } else {
                unset($attr[$def_i]);
            }
        }
    }