simple summation question

  Hello - a quick simple question, how would get a subTotal for someValue by
entity as shown below?
  I can get a grand total by using sum(/report/entity/row/someValue)..

  Thanks

   <report>
        <entity>
                <row>
                        <someValue>5</someValue>
                </row>
                <row>
                        <someValue>6</someValue>
                </row>
                <row>
                        <someValue>7</someValue>
                </row>
        </entity>

        <entity>
                <row>
                        <someValue>8</someValue>
                </row>
                <row>
                        <someValue>9</someValue>
                </row>
                <row>
                        <someValue>10</someValue>
                </row>
        </entity>
</report>



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