An asterisk got eaten in your post. You should start lines of code with a couple of spaces to make HN stop doing that.

  Set(1,2,3,4).map{ _ * 2 } // Set[Int] = Set(2, 4, 6, 8)

  BitSet(1,2,3,4).map{ _ * 2 } // BitSet = BitSet(2, 4, 6, 8)