CLHS: Section 2.1.4.3

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## 2.1.4.3 Invalid Characters

Characters with the constituent trait invalid cannot ever appear in a token except under the control of a single escape character. If an invalid character is encountered while an object is being read, an error of type reader-error is signaled. If an invalid character is preceded by a single escape character, it is treated as an alphabetic[2] constituent instead.

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