这个例程检查字符是否在A到Z之间，然后将其转换为小写字母并将它直接和其他字符比较。

This routine checks to see if a character is between a and Z, then converts it to lowercase and compares it; it compares other characters directly.

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返回一个字符串，该字符串中的所有字母都被转换为小写字母。

Returns a string where all alphabetic characters have been converted to lowercase.

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