Context-free in Scala

With less code than in Java the AbLanguage in Scala:

object AbLanguage {  
  def a = new AOpenedBase()
  
  protected class AOpenedBase {
    def a = new AOpened[AOpenedBase](this)    
    def b = {}
  }
  
  protected class AOpened[T](surroundingExpression : T) {
    def a = new AOpened[AOpened[T]](this)
    def b = surroundingExpression
  }
  
  def main(args : Array[String]) : Unit = {   
    a b;
    ((a a) b) b;
    a.a.b.b;
  }
}