返回一个字符串，该字符串中的所有字母都被转换为大写字母。
Returns a string where all alphabetic characters have been converted to uppercase.
它把这个传递给toupper函数，如果你们使用过，它表示的是把小写字母,转换为大写字母。
It's passing to this function called toupper, which if you've never used it it actually does what it says it makes it touppercase.
将选择的文字从大写字母转换为小写字母（反之亦然）；
Convert a selection of text from uppercase to lowercase (or vice versa).
将选择的文字从大写字母转换为小写字母（反之亦然）；
Convert a selection of text from uppercase to lowercase (or vice versa).
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