6.4.3 通用角色名称 http://c0x.coding-guidelines.com/6.4.3.html
通用字符名称不得指定除 0024 ($)、0040 (@) 或 0060 (`) 之外的短标识符小于 00A0 的字符，也不得指定 D800 到 DFFF（含）范围内的字符。
除了这样的限制不再“通用”之外，我想不出这种限制的充分理由。有人知道背景故事吗？
D800通过DFFF包含在内的不是有效的代码点；它们是高代理和低代理，只能在 UTF-16 编码中成对找到，以表示基本平面之外的代码点。
The other restriction avoids having a universal character name collide with a character which could be represented in the C character set, for the benefit of compilers which don't bother resolving universal character names into their unicode equivalents. So the compiler is under no obligation to recognize a + written as \u002B or to know that a and \u0061 represent the same name. ($, @ and ` are not valid in a C program outside of comments and character strings, so they do not require any special attention from the lexer.)
码位范围小于A0还包括控制字符和空格。 （C 不考虑\u00A0成为空白。）
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