Another sum question

I'm trying to sum attribute values by key. Multiple rows can have the same
key and I want to sum attributes of these rows. My XML looks like this:

<Row SORTKEY="1999D MN1 45759" GROSSAMT="+00000002917" />
<Row SORTKEY="1999D MN1 05133" GROSSAMT="+00000002978" />
<Row SORTKEY="1999D MN1 02057" GROSSAMT="+00000003115" />
<Row SORTKEY="1999D MN1 29354" GROSSAMT="+00000003580" />
<Row SORTKEY="2000D MN1 06944" GROSSAMT="+00000003730" />
<Row SORTKEY="2000D MN1 20763" GROSSAMT="+00000003914" />
<Row SORTKEY="2000D MN1 28002" GROSSAMT="+00000013962" />
					

For rows that have the matching first 10 chars of the SORTKEY, I want to get
a total for the GROSSAMT. So, for the first 4 rows, I want the total of the
GROSSAMTs. Same for the last 3 rows. 

Any help is greatly appreciated!

-Suraiya

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