The syntax is so archaic ... it really is anything but a thing of beauty.

    prev_balance = PREVIOUS(rec, order_by="Date").Running_Balance or 0
    prev_balance + $Amount

It's such a strange mix-up of stuff. PREVIOUS() is a macro or looks like one, right? But it is a function call assumingly. But then what's with .Running_Balance? Guess that also ties into a function. But why use different syntax for essentially the same thing? It seems rather weird to write code like that. What would the python variant be like? Or a DSL?

The syntax strangeness continues:

    rates = Rates.lookupRecords(Person=$Person, order_by="Rate_Start")
    rates.find.le($Date).Hourly_Rate

Does that really convey most easily what is wanted? It seems needlessly verbose. Why would it require both $Person and $Date? Should that not be easier to figure out in its own right? Is that efficient?

People need to remember that mentally, unless they use AI.