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How's the following function to update an immutable Map's values?
scala> def updateMap[A](map: Map[A, Set[A]], item: A, toAdd: A) = {
map.map(x => if(x._1 == item) (x._1, (x._2 + toAdd)) else (x._1, x._2))
}
scala> val map = Map(1 -> Set(2), 3 -> Set(4))
scala> updateMap(map, 1, 5)
res1...