protected function _getReferenceMapNormalized()
    {
        $referenceMapNormalized = array();

        foreach ($this->_referenceMap as $rule => $map) {

            $referenceMapNormalized[$rule] = array();

            foreach ($map as $key => $value) {
                switch ($key) {

                    // normalize COLUMNS and REF_COLUMNS to arrays
                    case self::COLUMNS:
                    case self::REF_COLUMNS:
                        if (!is_array($value)) {
                            $referenceMapNormalized[$rule][$key] = array($value);
                        } else {
                            $referenceMapNormalized[$rule][$key] = $value;
                        }
                        break;

                    // other values are copied as-is
                    default:
                        $referenceMapNormalized[$rule][$key] = $value;
                        break;
                }
            }
        }

        return $referenceMapNormalized;
    }