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Because transforming it into a map only "works" when your map function is injective.

what do you mean by injective? i’m a little confused by your answer here. 

also, i’d expect it to transform to a `List&lt;Pair&lt;A, List&lt;B&gt;&gt;&gt;`

i thought it had something to do with a terminal / collecting operation, while `.map{}` is just iterable -&gt; iterable

I was referring to <https://en.wikipedia.org/wiki/Injective_function.>

neat, good term - i was also suspecting something to do with distinctness but didn’t have the words for it