look ahead in SQL Server Reporting Services

[dove11]

I am using a stored procedure and returning rows to display in my report. I am grouping by a SOURCE field in the stored procedure. When the rows for this source are displayed on separate lines, I suppress the printing of the source on the subsequent lines by using the Previous function. In other words, if I have 5 rows, I print the source on the 1st row (occurrence) but not on rows 2-5. That works great. What I want to do now is sum the data columns for that source by adding a total line. I'm using the Previous function to check when the source changes, but I'm printing the total line on the first occurrence of the new source instead of the last (after row 1 instead of after row 5). I basically need to do a look ahead, determine how many rows I have, and then when I get the last row, print the total line. Is there a way of doing this within Reporting Services?