grouping and sum

how is it possible to group by the Customer Node to retrieve a sum from 
StepCharge?


<Portfilio>
<AsOf>11/01/2004</AsOf>
        <Trade>
                <TradeId>ED.TEST</TradeId>      
                <Customer>ABC</Customer>                
                <Step>
                   <MinFlowDate>11/30/2004</MinFlowDate>
                   <StepCharge>4</StepCharge>
                </Step>
                <Step>
                   <MinFlowDate>11/30/2005</MinFlowDate>
                   <StepCharge>6</StepCharge>
                </Step>
        </Trade>
        <Trade>
                <TradeId>ED.TESTFX1</TradeId>
                <Customer>AAA</Customer>        
                <Step>
                   <MinFlowDate>12/11/2004</MinFlowDate>
                   <StepCharge>3</StepCharge>
                </Step>
                <Step>
                   <MinFlowDate>12/11/2004</MinFlowDate>
                   <StepCharge>61</StepCharge>
                </Step>         
                <Step>
                   <MinFlowDate>12/11/2005</MinFlowDate>
                   <StepCharge>11</StepCharge>
                </Step>
        </Trade>        
               <Trade>
                <TradeId>ED.TESTFX1</TradeId>
                <Customer>AAA</Customer>        
                <Step>
                   <MinFlowDate>12/11/2004</MinFlowDate>
                   <StepCharge>3</StepCharge>
                </Step>
                <Step>
                   <MinFlowDate>12/11/2004</MinFlowDate>
                   <StepCharge>61</StepCharge>
                </Step>         
                <Step>
                   <MinFlowDate>12/11/2005</MinFlowDate>
                   <StepCharge>11</StepCharge>
                </Step>
        </Trade>        

</Portfilio>




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