Using report variables to calculate subtotals in xml side

Hello,

I'm having trouble grasping how to implement report variables, break 
headers, and output them in xml.

Consider I have the following output:

<Detail>
/.../

<FieldDetails>
	<Output>
		<Line bgcolor="iif(r.detailcnt%2,'0xe5e5e5','white')">
			<field value="r.field1" width="10" align="left" col="1"/>
			<literal width="1"/>
			<field value="r.field2" width="10" align="center" col="2"/>
			<literal width="1"/>
			<field value="val(r.field3)" format="'$%.2d'" width="35" 
align="left" col="3"/>
		</Line>
		<HorizontalLine size="4" bgcolor="'white'"/>
	</Output>
</FieldDetails>

</Detail>

And I want r.field3 to be summed up and have a subtotal displayed at 
the end of this output.  First, where do I put the following line to 
calculate the subtotal?
	<Variable name="r.subtotal" value="r.total_fees" type="sum" />
Do I place it between the <FieldDetails> tags?

And then, where and how can I display the actual subtotal?  The syntax 
would be very much like this, right?  I'd like it to align with the 
column 3 referenced between the Detail tags.
				<field value="r.subtotal" width="10" align="right" col="3"/>


-Al


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