public static function parseExpressionXml(SimpleXMLElement $xml){
        $returnValue = self::parseElementXml($xml);
        $value = trim($xml);
        $expressions = array();
        foreach($xml->children() as $child){
            $expressions[] = self::parseExpressionXml($child);
        }
        if(count($expressions)){
            $returnValue['expressions'] = $expressions;
        }
        if(strlen($value)){
            $returnValue['value'] = $value;
        }
        return $returnValue;
    }