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From: Jesper Harder <harder@myrealbox.com>
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Subject: Re: What does the coding system nil mean?
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Eli Zaretskii <eliz@elta.co.il> writes:

>> So, can anyone explain what a nil coding system means?
>
> It means, quite naturally, that no code-conversion should take
> place.  That is, the original text or string are left unaltered.

Then nil isn't really a coding system, but just a value that some
coding system related functions happen to interpret in a certain way.

Ah, it turns out that I was confused because I hadn't read the
documentation for `coding-system-p' carefully enough:

    Return t if OBJECT is nil or a coding-system.

Though, I think it's a bit odd for a predicate called
`coding-system-p' to return t for an object that is _not_ in fact a
coding system.