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kotlinlang

This isn't Kotlin specific, more of a general functional question. 

If I have a collection (items) and I want to do a 1-1 transformation, I'd use map `items.map { mapMethod }` to do a 1-1 transformation on `items` 
If I want to do a 1-N transformation, I'd use flatMap to turn each item into 1 or more newItems 

What if I need to turn `items` of length N into M new items (M &lt; N) where each newItem could be made from 1 or more item? Is there a functional way to do that?