Invalid Characters (ANSI and GNU Common Lisp Document)

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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.