@Override
  public void replaceExprent(Exprent oldExpr, Exprent newExpr) {
    if (oldExpr == left) {
      left = newExpr;
    }
    if (oldExpr == right) {
      right = newExpr;
    }
  }
@Override
  public void replaceExprent(Exprent oldExpr, Exprent newExpr)