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>