class Matrix(val elements:List[List[Int]]) {

  // --- Snip ---

  def size = (elements.length, elements.head.length)

  def +(that:Matrix) = {
    require(this.size == that.size)
    new Matrix(this.elements zip that.elements map(sum))
  }

  private def sum(t:(List[Int], List[Int])) : List[Int] = t._1 zip t._2 map(sum)

  private def sum(t:(Int, Int)) : Int = t._1 + t._2

  def *(c:Int) = new Matrix(elements.map(_.map(_ * c)))

}