InString[n] is a global object that is assigned to be the text of the n input line.
InString[n] gives the string that Mathematica read for the n input line. The string includes all intermediate newlines in the input, but not the newline at the end.
The value of InString[n] is assigned after the input is verified to be syntactically correct, and after any function given as the value of $PreRead has been applied.
InString[ ] gives the text of the last input line.
InString[-k] gives the text of the input k lines back.
See The Mathematica Book: Section 1.3.3 and Section 2.13.1.
See also: In, $SyntaxHandler.
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