The alternative is to have an additional control in the Right pane to specify the order. But that also comes with its own issues. Especially for transforms like Join which can have any number of inputs.
Shame, I got excited then as this would allow me to dispense with nested joins.
I still think that my original suggestion works best:
This would only apply to Subtract and possibly any similar ‘non-symmetrical’ transforms.
This isn’t a major deal for me as I have a workaround and I’m not sure how many others would like/need this option so I imagine it wouldn’t be high priority for you.
You can add a New Col to both lists. Name the col “List”.
For List A the new col can have the value A for list B the col value B.
Stack the Data
Use the unique transform to concat the list col.
The result will tell you which data rows are in list A only, list B, or in both.