I have worked through the Kotlin Koans a couple of times. I really enjoyed the collections exercises. Is there anything else out there that’s similar?

When I search I see a lot of introductory articles, but not much on intermediate stuff using collections and filter/map/flatmap/any/all/min/max/groupby/partition/fold/reduce/etc/etc.

I need lots more practice, and I find it motivating to have problems to solve. Any good and fun resources?