[tex-k] Index Entry of \not in The TeXbook

[Karl Berry]

    |\not| ( $\not$ ), @133, +358, @436--437.

Hi Sebastian - thanks for the report, but Knuth has said this is
intentional. In errata.three
(https://www.ctan.org/tex-archive/systems/knuth/dist/errata/), there is:

  \bugonpage A472, the entry for {\tt\char`\\not} (2/12/87)

  \eightpoint\noindent
  [The overprinting here is intentional, since {\tt\char`\\not} is a
  character of width zero. More than a dozen people have reported this
  as an error, but it is not!]

--best, karl.