def maxList[T <% Ordered[T]](elements: List[T]): T =
  elements match {
    case List() =>
      throw new IllegalArgumentException("empty list!")
    case List(x) => x
    case x :: rest =>
      val maxRest = maxList(rest) // (orderer) is implicit
      if (x > maxRest) x // orderer(x) is implicit
      else maxRest
  }