declare namespace rpt="http://www.example.com/Report";

(: insert-start :)
declare variable $input-context external;
(: insert-end :)

let $orders := $input-context/purchaseOrder
                 [@orderDate ge xs:date("1999-09-01")
                  and @orderDate le xs:date("1999-12-31")] 
let $items := $orders/items/item
let $zips := distinct-values($orders/billTo/zip)
let $parts := distinct-values($items/@partNum)
return
 <rpt:purchaseReport>
  <rpt:regions>
    {
     for $zip in $zips
     order by $zip
     return
      <rpt:zip code="{$zip}">
       {
        for $part in $parts
        let $hits := $orders[ billTo/zip = $zip and items/item/@partNum = $part]
        let $quantity := sum($hits//item[@partNum=$part]/quantity)
        where count($hits) > 0
        order by $part
        return
         <rpt:part number="{$part}" quantity="{$quantity}"/>
       }
      </rpt:zip>
    }
  </rpt:regions>
  <rpt:parts>
   {
     for $part in $parts
     return
      <rpt:part number="{$part}">
       {
         string($items[@partNum = $part]/productName)
       }
      </rpt:part>   
   }
  </rpt:parts>
</rpt:purchaseReport>