How to sort the xml element based on data in wso2esb

I am using wso2esb. I wish to transform the data using wso2esb i have done the issue is while response i am getting data in descending manner how would i arrange in to ascending way .

<response>
    <customer-details>
        <cusfirstname>Empire Burlesque</cusfirstname>
        <delarname>Bob Dylan</delarname>
        <cusno>254</cusno>
    </customer-details>
    <customer>
        <cusId>6</cusId>
        <customername>Bonnie Tyler</customername>
        <country>UK</country>
        <company>CBS Records</company>
        <price>9.90</price>
        <year>1988</year>
    </customer>
    <customer>
        <cusId>8</cusId>
        <customername>Dolly Parton</customername>
        <country>USA</country>
        <company>RCA</company>
        <price>9.90</price>
        <year>1982</year>
    </customer>
    <customer>
        <cusId>3</cusId>
        <customername>Gary Moore</customername>
        <country>UK</country>
        <company>Virgin records</company>
        <price>10.20</price>
        <year>1990</year>
    </customer>
    <customer>
        <cusId>5</cusId>
        <customername>Eros Ramazzotti</customername>
        <country>EU</country>
        <company>BMG</company>
        <price>9.90</price>
        <year>1997</year>
    </customer>
    <customer>
        <cusId>7</cusId>
        <customername>Bee Gees</customername>
        <country>UK</country>
        <company>Polydor</company>
        <price>10.90</price>
        <year>1998</year>
    </customer>
    <customer>
        <cusId>1</cusId>
        <customername>Dr.Hook</customername>
        <country>UK</country>
        <company>CBS</company>
        <price>8.10</price>
        <year>1973</year>
    </customer>
    <customer>
        <cusId>2</cusId>
        <customername>Rod Stewart</customername>
        <country>UK</country>
        <company>Pickwick</company>
        <price>8.50</price>
        <year>1990</year>
    </customer>
    <customer>
        <cusId>4</cusId>
        <customername>Andrea Bocelli</customername>
        <country>EU</country>
        <company>Polydor</company>
        <price>10.80</price>
        <year>1996</year>
    </customer>
    <customer>
        <cusId>12</cusId>
        <customername>Percy Sledge</customername>
        <country>USA</country>
        <company>Atlantic</company>
        <price>8.70</price>
        <year>1987</year>
    </customer>
    <customer>
        <cusId>9</cusId>
        <customername>Savage Rose</customername>
        <country>EU</country>
        <company>Mega</company>
        <price>10.90</price>
        <year>1995</year>
    </customer>
    <customer>
        <cusId>11</cusId>
        <customername>Many</customername>
        <country>USA</country>
        <company>Grammy</company>
        <price>10.20</price>
        <year>1999</year>
    </customer>
    <customer>
        <cusId>10</cusId>
        <customername>Kenny Rogers</customername>
        <country>UK</country>
        <company>Mucik Master</company>
        <price>8.70</price>
        <year>1995</year>
    </customer>
</response>

I wish to arrange the customer tag data in ascending order based on cusId so how would i arrange this I tried with xquery but i couldn't find the any function regarding element sorting , cusId should be appear in 1,2,3,...

How would i achieve this i know there is no function in xpath for this.

thanks in advance.