type Occurrences = List[(Char, Int)]

  def subtract(x: Occurrences, y: Occurrences): Occurrences = {
    val map = y.toMap
    x.foldRight(List[Char, Int]()){
      case ((c, i), xs) if map.contains(c) => (c, 9) :: xs
      case (_, xs) => xs
    }
  }