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fun <T, M> Iterable<T>.countBy(transformer: (T) -> M) : Map<M, Int> {
val emptyMap = mapOf<M, Int>()
return this.fold(emptyMap) {accumulator, item ->
val transformedItem = transformer(item)
accumulator + Pair(transformedItem, accumulator.getOrElse(transformedItem, {0}) + 1)
}
}
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