[xsl] for-each and summing based on group

within a for-each there might be various sets of values which produce the 
following grid, where val is equal to a number

a  val val val
a  val val val
a  val val val
b  val val val
b  val val val
c  val val val
c  val val val

what is the best way to insert a sum line for the first
column a, one for b, and one for c.

is a nested for each needed? or checking to see if the following value is 
not equal to the preceding, then then summing?

thanks,
Dmitri

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