Getting subtotals of an XML file

[Dracosms]

My current XML file looks like this:

<Order>
  <Customer>customer1 </Customer>
  <Amount> 20 </Amount>
</Order>
<Order>
  <Customer>customer1 </Customer>
  <Amount> 90 </Amount
</Order>
<Order>
  <Customer>customer2 </Customer>
  <Amount> 50 </Amount
</Order>

What kind of XSL expression should I use to get the (sub)totals of each customer ?
I know I can get the total of all order by " <xsl:value-of select='format-number(sum(//Order/Amount), "#######.00")'/> "

Can anyone help me ?

Greetz,

Dracosms