E.g. UPPER($A) is just a function that calls $A.upper(). And $A is just syntactic sugar over rec.A.

PREVIOUS() is only special in that it automatically creates an index, which is important for efficiency. But generally you can look up a set of records and write multi-line Python code to do whatever you want with it and return a value. And it all runs in a sandbox with no access to anything except this document's data.