inline fun <T, K> Sequence<T>.groupingBy( crossinline keySelector: (T) -> K ): Grouping<T, K>
Platform and version requirements: Kotlin 1.1
Creates a Grouping source from a sequence to be used later with one of group-and-fold operations using the specified keySelector function to extract a key from each element.
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Licensed under the Apache License, Version 2.0.
https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.sequences/grouping-by.html