请补充函数fun()，该函数的功能是：把从主函数中输入的字符串str2接在字符串str1的后面。
例如：str1=“How do”，str2=“ you do?”，结果输出：How do you do?
-Please add a function fun (), the function of the functions are: to enter from the main function of the string str2 connected to the back of the string str1. For example: str1 = " How do" , str2 = " you do?" , The resulting output: How do you do?
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