检查是否返回了以前的状态，以便用户可以再次借阅图书

class Book:
    def __init__(self, title, is_borrowed=False):
        self.title = title
        self.is_borrowed = is_borrowed

    def borrow(self):
        if not self.is_borrowed:
            self.is_borrowed = True
            print(f"{self.title} has been successfully borrowed.")
        else:
            print(f"Sorry, {self.title} is currently borrowed.")

    def return_book(self):
        if self.is_borrowed:
            self.is_borrowed = False
            print(f"{self.title} has been successfully returned.")
        else:
            print(f"Error: {self.title} was not borrowed.")

# 示例使用
book1 = Book("The Great Gatsby")
book1.borrow()  # 输出: The Great Gatsby has been successfully borrowed.
book1.return_book()  # 输出: The Great Gatsby has been successfully returned.
book1.borrow()  # 输出: The Great Gatsby has been successfully borrowed.