aspect Interactive {
  public boolean Start.interactive() {
    return getExp().interactive();
  }

  /*
    Returns true if this subtree contains any "Ask User" construct.
  */
  public boolean Exp.interactive() {
    return false;         // Default result
  }

  public boolean MulExp.interactive() {
    return getLeftExp().interactive() || getRightExp().interactive();
  }

  public boolean DivExp.interactive() {
    return getLeftExp().interactive() || getRightExp().interactive();
  }

  public boolean LetExp.interactive() {
    boolean soFar = false;
    for (int k = 0; k < getNumBinding(); k++) {
      soFar = soFar || getBinding(k).interactive();
    }
    return soFar || getExp().interactive();
  }

  public boolean Binding.interactive() {
    return getExp().interactive();
  }

  public boolean AskUserExp.interactive() {
    return true;
  }
}