protected function _getClassIndices($class)
    {
        $indices = array ();
        if ($class == self::CLASS_LETTER) {
            $indices = array_merge(
                $this->_getClassIndices(self::CLASS_LOWER),
                $this->_getClassIndices(self::CLASS_UPPER)
            );
            sort($indices);
        } else {
            if (isset ($this->_tokenIndices[$class])) {
                $indices = $this->_tokenIndices[$class];
            }
        }

        return $indices;
    }