This compute block computes the value of Perish from the values for
the Meat/Dairy department and the Produce department. Because the variables
Sales and Department collectively define these columns, there is no way to
identify the values to PROC REPORT by name. Therefore, the assignment statement
uses column numbers to unambiguously specify the values to use. Each time
PROC REPORT needs a value for Perish, it sums the values in the third and
fourth columns of that row of the report.