[tex-k] Bug-report for the TeXbook: Not all non-primitive control-sequences are defined, ultimately, in terms of the primitive ones.

Don Hosek don.hosek at gmail.com
Wed Dec 14 20:59:43 CET 2022

> In other words, the control sequence |null_cs| with no characters to its
> name is always output to the world as the two primitive CSes,
> "\csname\endcsname".  So I guess the pertinent question is, can |null_cs|'s
> meaning be redefined?  If not, isn't it "a primitive" CS?
Any control sequence in TeX an be redefined. Most notably, LaTeX redefines
the meaning of \end (among others).

Being a primitive means that the control sequence executes something
directly in TeX rather than expanding into a sequence of tokens. null_cs
has special handling for output and its location is used for other purposes
in the hash table (it’s a boundary between the single character csnames and
multiple-character csnames), but it is not a primitive.

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