reserved message code 保留消息码
Those characters that do not map will be substituted by a reserved code point.
那些没有映射的字符将通过预留的代码点来代替。
Every code page has reserved at least one code point for substitution.
每一个代码页至少预留了一个代码点,以便进行替代。
As only 27 tokens are available for elements -- 5 bits, with the values 0 through 4 being reserved -- complex vocabularies need to be multiplexed by organizing token sets into separate code pages.
由于元素的标记只有27种——共占用5位,其中值从0到4为保留标记——复杂名词则需要进行复合,将标记集组织到不同的代码页中。
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