（这里的字节并不是像UTF-16中那样简单的Unicode编码点（unicode code point）；它应用了一些位变换（bit-twiddling）。）中文字符比如“中”则占用了3个字节。
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Returns the character (Unicode code point) at the specified index.
返回指定索引的字符(Unicode码的位置)。
The only collation URI defined by XPath 2.0 is for the Unicode code point collation, according to which strings are compared by the Unicode code points of their individual characters.
XPath 2.0仅定义了一种排序uri，即Unicode代码点排列法，字符串根据每个字符的Unicode代码点进行比较。
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