Auto-numbering tags without incrementing

Is there a way to use auto-numbering tags without them incrementing every time? I though I had figured this out with the sub-numbering, but that’s not quite what I’m after. For instance, how could I get this output for the Part #'s?

Part 1, Chapter 1
Part 1, Chapter 2
Part 1, Chapter 3
Part 2, Chapter 1
Part 2, Chapter 2

I know how to use the <$rst_ bit to reset numbering for the chapters, and I can create named streams (like <$n:chapter>), but I can’t figure out how to do the part numbering as it is above.