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From: Jesper Harder <harder@myrealbox.com>
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Subject: What does the coding system nil mean?
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  (coding-system-p nil) => t

OK, so nil is a coding system.

But I can't find any documentation about what it means to supply nil
to functions that take a coding system (`encode-coding-string' etc.),
and `describe-coding-system' refuses to describe the nil coding
system.

Hmm, `encode-coding-string' promises to set `last-coding-system-used'
... yet if you supply nil, it doesn't change the previous value of the
variable.

So, can anyone explain what a nil coding system means?