Can you please post some snapshots of what are you trying to achieve. We don't have native ISLASTOCCURENCE functionality , the closest we have is LastNonBlank - as an aggregation functionality and you can also change the summary settings of a line item to Last Non Blank - Explore these options and see if this helps
If not, feel free to post a snip here about the expected outcome - will surely try to help
I suppose that you mean like this, don't you?
I set three modules.
1. Choose from List
-- dimension: numbered list (#1-20), Line item 'Choice' formatted as the List Item Choice(A-D)
-- dimension: numbered list(#1-20) & Choice(A-D)
Line Item 'Which' is described below
numbered list has build-in top level and its summary method set as 'Lastnonblank'
3. What is the last row
-- dimension: Choice(A-D)
Line Item 'the last row' = process.which
I hope this works in your model.