扩展字符集是基本字符集(2.2)的超集。
The extended character set is a superset of the basic character set (2.2).
一个字节应该至少具有足够表示运行时基本字符集的任意成员的大小，并且由实现定义个数的连续位构成。
A byte is at least large enough to contain any member of the basic execution character set and is composed of a contiguous sequence of bits, the number of which is implementation-defined.
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