However, this would only work for the default values of ‘packages’.  In
other cases, users would have to add (file-system-packages …) explicitly
by themselves, which is not great.

Alternately, we could turn ‘%base-packages’ into a macro that expands
to something like:

  (gimme-the-base-packages this-operating-system)

but that wouldn’t be great because now you’d be unable to refer to
‘%base-packages’ like a regular variable, outside the lexical context of
an ‘operating-system’ form.

Thoughts?

Ludo’.