class X(val x: Int) { 
  def freak(other: X) = new X(x+other.x)
  def +(other: X) = new X(x+other.x) 
}

var x = new X(1)
x += new X(2)
x.x => 3
x freak= new X(3)
error: reassignment to val
       x freak= new X(3)
              ^