今日学习：Exercise 19: Functions And Variables
Exercise 20: Functions And Files
1.【Write at least one more function of your own design, and run it 10 different ways.】（不懂）
2.seek函数（不懂）
3.“=+” 用于循环结构
e.g.
【
current_line = 1
while (current_line <= 3):
print_a_line(current_line, current_file)
current_line += 1
】