Should `\texdimennc` and `\texdimennd` and their "up and down" variants be left undefined for `xetex` and `e(u)ptex`?

[jfbu]

Units nc and nd are not available with the XeTeX and e(u)pTeX engines. But the package still provides associated unit conversion macros. Should it keep doing so?

• con: the units are not available in these engines,
• pro: the macros will work correctly and not raise any specific error, so they could be useful in typesetting with these engines some article devoted to TeX dimensions, or needing to illustrate some conversions. Admittedly such article would probably also need to illustrate inputs with nc/nd units which would be possible only via extra mark-up and mimicking TeX exact behaviour, this is possible using xintexpr.

The only problem here is that \dimexpr\texdimennc{1pt}nc\relax will crash simply from engine not recognizing unit, but \texdimennc{1pt} has no issue:

$ rlwrap euptex
This is e-upTeX, Version 3.141592653-p4.1.0-u1.29-230214-2.6 (utf8.uptex) (TeX Live 2024) (preloaded format=euptex)
 restricted \write18 enabled.
**texdimens
entering extended mode
(/usr/local/texlive/2023/texmf-dist/tex/generic/texdimens/texdimens.tex)
*\message{\texdimennc{1pt}, \texdimenncup{1pt}, \texdimenncdown{1pt}}
0.07811, 0.07811, 0.0781
*\bye
No pages of output.
Transcript written on texdimens.log.

This looks to me as only a documentation issue.

Relates: latex3/latex3#1217