Sometimes in test conditions in rules, there may be a need to
test for sums or averages of a collection of values, and these
kinds of aggregated values may not be directly available as input
facts. For example, in an expense report, you may want to test if
the total expenses are above a certain value and then take some
action. This is easy if the total value is available directly in
the expense report header. What if the total value is not available
in the expense report...
