I need to write a complicated  X query. The Query is
1] which lists 2 most popular pairs of products bought together.

and XML file is

<e-store>
   <product id="cre30" name="Creative ZEN VISION"
price="255.99">Creative White 30GB USB 2.0 ZEN VISION Model
70PF115100011 - Retail</product>
   <product id="ipod30" name="Apple iPod" price="236.99">Apple new
iPod video Black 30GB Model MA446LL/A - Retail</product>
   <product id="sam2" name="Samsung Black" price="154.99"> SAMSUNG
Black 2GB USB 2.0 Audio Player with Built-in Speakers Model YP-K5JQB -
Retail</product>
   <product id="tosh60" name="Toshiba Piano Black" price="175.99">
Recertified: TOSHIBA Piano Black 60GB USB 2.0 Portable Media Player
Model MES60VK</product>
   <product id="san4" name="SanDisk Sansa" price="135.99"> SanDisk
Sansa e260 Black 4GB USB 2.0 MP3 Player Model SDMX44096A70 - Retail</
product>
   <product id="msft30" name="Microsoft Zune Black" price="223.99">
Microsoft Zune Black 30GB Hi-Speed USB, Wi-Fi(802.11 b/g) Large color
screen Digital media player Model JS8-00001 - Retail</product>
   <product id="iaud2" name="iAUDIO Black" price="162.99">iAUDIO Black
2GB USB 2.0 Portable Video Player Model D2-2048BL - Retail</product>
   <product id="arch30" name="ARCHOS Silver" price="255.99"> ARCHOS
Silver 30GB USB 2.0 High-Speed Device (compatible USB 1.1): Mass
Storage class (MSC) and Media Transport Protocol (MTP)</product>
   <product id="ipod80" name="Apple iPod video" price="324.99"> Apple
new iPod video Black 80GB Model MA450LL/A - Retail</product>
   <product id="ipod2" name="Apple new iPod nano" price="142.99">Apple
new iPod nano Silver 2GB Model MA477LL/A - Retail</product>

   <warehouse address="NY">
      <product-item product-id="cre30" quantity="5"/>
      <product-item product-id="ipod30" quantity="3"/>
      <product-item product-id="sam2" quantity="3"/>
      <product-item product-id="tosh60" quantity="2"/>
      <product-item product-id="san4" quantity="2"/>
   </warehouse>

   <warehouse address="TX">
      <product-item product-id="cre30" quantity="5"/>
      <product-item product-id="sam2" quantity="3"/>
      <product-item product-id="san4" quantity="20"/>
      <product-item product-id="msft30" quantity="1"/>
      <product-item product-id="iaud2" quantity="2"/>
      <product-item product-id="ipod2" quantity="4"/>
   </warehouse>

   <client name="John" address="Buffalo, NY">
      <order order-date="2007-04-15" shipping-date="2007-04-18">
         <item product-id="cre30" quantity="1"/>
	 <item product-id="sam2" quantity="2"/>
	 <item product-id="arch30" quantity="1"/>
	 <item product-id="ipod80" quantity="4"/>
      </order>
      <order order-date="2007-04-15" shipping-date="2007-04-17">
         <item product-id="cre30" quantity="1"/>
	 <item product-id="ipod2" quantity="2"/>
	 <item product-id="sam2" quantity="2"/>
      </order>
      <order order-date="2007-04-23">
         <item product-id="msft30" quantity="6"/>
	 <item product-id="sam2" quantity="2"/>
	 <item product-id="ipod30" quantity="2"/>
      </order>
   </client>

   <client name="Larry" address="Albany, NY">
      <order order-date="2007-04-18" shipping-date="2007-04-20">
         <item product-id="cre30" quantity="1"/>
	 <item product-id="ipod2" quantity="1"/>
	 <item product-id="sam2" quantity="1"/>
      </order>
      <order order-date="2007-04-18" shipping-date="2007-04-20">
         <item product-id="sam2" quantity="1"/>
	 <item product-id="ipod30" quantity="1"/>
	 <item product-id="san4" quantity="1"/>
      </order>
      <order order-date="2007-04-22">
         <item product-id="san4" quantity="1"/>
	 <item product-id="sam2" quantity="2"/>
	 <item product-id="iaud2" quantity="2"/>
      </order>
   </client>

   <client name="Liz" address="Niagara Falls, NY">
      <order order-date="2007-03-12" shipping-date="2007-03-14">
	 <item product-id="san4" quantity="1"/>
	 <item product-id="sam2" quantity="1"/>
	 <item product-id="arch30" quantity="3"/>
      </order>
      <order order-date="2007-04-24">
	 <item product-id="iaud2" quantity="1"/>
      </order>
   </client>
</e-store>


Actually I am new to X query and it is hard to implement using X Path.
Any help will be really appreciated .

Regards