// Textbook fragment 07.32
/** Print out the expression stored in an arithmetic expression tree.  */
public class PrintExpressionTour extends EulerTour<ExpressionTerm, String> {
  public String execute(BinaryTree<ExpressionTerm> T) {
    init(T);
    System.out.print("Expression: ");
    eulerTour(T.root());
    System.out.println();
    return null;        // nothing to return
  }
  protected void visitLeft(Position<ExpressionTerm> v, TourResult<String> r) {
    if (tree.isInternal(v)) System.out.print("("); }
  protected void visitBelow(Position<ExpressionTerm> v, TourResult<String> r) {
    System.out.print(v.element()); }
  protected void visitRight(Position<ExpressionTerm> v, TourResult<String> r) {
    if (tree.isInternal(v)) System.out.print(")"); }
}