<result>
  {
    for $item in doc("items.xml")//item_tuple
    let $b := doc("bids.xml")//bid_tuple[itemno = $item/itemno]
    let $z := max(for $z in $b/bid return number($z))
    where $item/reserve_price * 2 < $z
    return
        <successful_item>
            { $item/itemno }
            { $item/description }
            { $item/reserve_price }
            <high_bid>{$z }</high_bid>
         </successful_item>
  }
</result>