RE : xsl:sort blocks of element with an attribute value

Hi
 
        Could someone help me with this.  
 
        I would like to sort each ?NetPrice? block by the attribute value
?netAmount? (ascending) of each block.
 
        Here is my XML:
 
<GetProductOptionsResponse xmlns="http://www.accovia/TRIP2004A/01";> 
    <Header language="en" version="5.1.0"/>
        <DetailedProduct availabilityStatus="available">        
        <ProductOptionGroup description="OPTION GROUP 1">            
            <ProductOption description="option A">
               <NetPrice currency="EUR" netAmount="2.27"
priceReferenceID="HTLAVO13XAAX" quantity="1"/>
               <NetPrice currency="EUR" netAmount="-45.09"
priceReferenceID="HTLAVO13XAAX" quantity="1"/>
               <TravelerPrice age="10" travelerId="003">
                    <PriceSummary baseAmount="0.00"/>
               </TravelerPrice>
            </ProductOption>
            <ProductOption description="option B">
                <NetPrice currency="EUR" netAmount="2.27"
priceReferenceID="HTLAVO13XAAX" quantity="1"/>
                <NetPrice currency="EUR" netAmount="-45.09"
priceReferenceID="HTLAVO13XAAX" quantity="1"/>
               <TravelerPrice age="10" travelerId="003">
                    <PriceSummary baseAmount="0.00"/>
               </TravelerPrice>
            </ProductOption>
        </ProductOptionGroup>
        <ProductOptionGroup description="OPTION GROUP 2">            
            <ProductOption description="option Z">
                <NetPrice currency="EUR" netAmount="5.55"
priceReferenceID="BHTLAVO13XAAX" quantity="1"/>
                <NetPrice currency="EUR" netAmount="-145.09"
priceReferenceID="BHTLAVO13XAAX" quantity="1"/>
               <TravelerPrice age="99" travelerId="001">
                    <PriceSummary baseAmount="1.00"/>
               </TravelerPrice>
            </ProductOption>
        </ProductOptionGroup>
        </DetailedProduct>
</GetProductOptionsResponse>
 
        I have tried many many ways without success.
        
Thanks in advance for your help
Stephan