I had no idea...

Crazy.scala

trait Invert extends AddOperation {
  override abstract def add(a: Int, b: Int): Int = super.add(a * -1, b * -1)
}

trait MultDiv extends AddOperation {
  override abstract def add(a: Int, b: Int): Int = super.add(a * 2, b / 2)
}

trait Inc2Dec extends AddOperation {
  override abstract def add(a: Int, b: Int): Int = super.add(a + 2, b -1)
}

class AddOperation {
  def add(a: Int, b: Int): Int = a + b
}

object Crazy extends App {
  val a = new AddOperation
  println(a.add(10, 11)) //21
  val b = new AddOperation with Inc2Dec 
  println(b.add(10, 11)) //22
  val c = new AddOperation with Inc2Dec with MultDiv
  println(c.add(10, 11)) //26
  val d = new AddOperation with MultDiv with Inc2Dec
  println(d.add(10, 11)) //29
  val e = new AddOperation with Invert with Inc2Dec
  println(e.add(10, 11)) //-22
  val f = new AddOperation with Inc2Dec with Invert
  println(f.add(10, 11)) //-20
}