Perhaps GUIX_BUILD (suffix with _OPTS or some such?) is not the best name and I'm not certain if it shouldn't be stdout (why are we using error port for "normal" communication? Emacs?), but I think it might prove useful if user keeps being reminded his environment is polluted or not as his intentions might have been, or it could go "only" into guix|guix --help...

To doc or not to doc,
Drp
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