The first aggregate example is

define function Factorial(X Integer):
  (expand Interval[1, X]) R
    aggregate T: Coalesce(T, 1) * R 

While this is pretty simple, there are a few things that aren't immediately obvious upon first read:

• What does R stand for? At first I thought "Result" but that definitely set me off on the wrong foot when trying to understand the query because R is definitely not the result of the aggregation in this case.
• What does T stand for? Is it "Total"? In some aggregations (like summations) that makes sense, but maybe not in a factorial?
• What does T actually do?  Since the syntax of aggregation hasn't been introduced, the reader needs to try to figure out what the argument after "aggregate" actually means.
• What's that Coalesce there for?  I know it's because T is null the first time, but I had to think about it.  Wouldn't "starting 1" be the recommended approach?

In addition, using expand Interval[1,X] complicates the example since the author needs to remember how expand works with intervals when no per is specified...

So... since this is the first introduction of aggregate, I think it makes sense to make the example as simple as possible while also following recommended practices.  To that end, I'd suggest something more like:

define function FactorialOneToFive:
  ({1,2,3,4,5}) Num
    aggregate Result starting 1: Result * Num 

If you want to encourage single character aliases, then I'd use N and R as someone might be able to guess number and result.