https://static.javadoc.io/org.scala-lang/scala-library/2.12.6/scala/collection/mutable/SetBuilder.html https://static.javadoc.io/org.scala-lang/scala-library/2.12.6/scala/collection/mutable/MapBuilder.html seen in https://github.com/akka/akka/blob/a0e79a0542dc0fc98bf8431a1cfd1a454f45b282/akka-cluster/src/main/scala/akka/cluster/Reachability.scala#L75-L77 https://github.com/akka/akka/blob/a0e79a0542dc0fc98bf8431a1cfd1a454f45b282/akka-cluster/src/main/scala/akka/cluster/protobuf/ClusterMessageSerializer.scala#L446
