Funny pattern matching

ToRome.scala

object ToRome {

  sealed class RomeDigit(val value: Int)
  case object I extends RomeDigit(1)
  case object V extends RomeDigit(5)
  case object X extends RomeDigit(10)
  case object L extends RomeDigit(50)
  case object C extends RomeDigit(100)
  case object D extends RomeDigit(500)
  case object M extends RomeDigit(1000)

  def toRome(x: Int): List[RomeDigit] = x match {
    case x if x >= M.value => M :: toRome(x - M.value)
    case x if x >= M.value - C.value => C :: M :: toRome(x - (M.value - C.value))
    case x if x >= D.value => D :: toRome(x - D.value)
    case x if x >= D.value - C.value => C :: D :: toRome(x - (D.value - C.value))
    case x if x >= C.value => C :: toRome(x - C.value)
    case x if x >= C.value - X.value => X :: C :: toRome(x - (C.value - X.value))
    case x if x >= L.value => L :: toRome(x - L.value)
    case x if x >= L.value - X.value => X :: L :: toRome(x - (L.value - X.value))
    case x if x >= X.value => X :: toRome(x - X.value)
    case x if x >= X.value - I.value => I :: X :: toRome(x - (X.value - I.value))
    case x if x >= V.value => V :: toRome(x - V.value)
    case x if x >= V.value - I.value => I :: V :: toRome(x - (V.value - I.value))
    case x if x >= I.value => I :: toRome(x - I.value)
    case 0 => Nil
    case _ => throw new IllegalArgumentException
  }

  def main(args: Array[String]) {
    1 to 10 foreach { x =>
      println(toRome(x))
    }
    println(toRome(15))
    println(toRome(19))
    println(toRome(8))
    println(toRome(64))
    println(toRome(63))
    80 to 100 foreach { x =>
      println(toRome(x))
    }
    39 to 55 foreach { x =>
      println(toRome(x))
    }
    println(toRome(1993))
    println(toRome(2010))
  }

}