I have a document where I want to group based on values several levels deep:

<SalesOrder ...>
    <LineItems>
        <LineItem Id="1" DeliveryAddressId="Shipping" DeliveryMethodId="1" 
SupplierId="A" />
        <LineItem Id="2" DeliveryAddressId="Shipping" DeliveryMethodId="2" 
SupplierId="A" />
        <LineItem Id="3" DeliveryAddressId="Shipping" DeliveryMethodId="1" 
SupplierId="B" />
        <LineItem Id="4" DeliveryAddressId="Billing" DeliveryMethodId="2" 
SupplierId="B" />
    </LineItems>
</SalesOrder>

The above example should ideally end up in four purchase orders because of 
the combination of delivery address, method, and supplier.  For example:

<PurchaseOrder SupplierId="A" DeliveryMethod="1">
    <DeliveryAddress Id="Shipping" .. />
    <LineItems>
        <LineItem Id="1" />
</PurchaseOrder>

At this point I've gotten to a point where I can build out single line-item 
purchase orders based on this information.   Ideally I want to aggregate 
based on the three values using XSLT.

Has anyone seen this scenario before?  Are there any good resources that you 
could share to help me understand what functions/approaches to take to 
resolve this?

Thanks,
Colin