However, looking at the allocation routines in (gnu build accounts), I
think that this would forcefully set ‘gdm’ to 900/900 on existing
installations, even if 900 is already used by another account:

--8<---------------cut here---------------start------------->8---
scheme@(gnu build accounts)> (allocate-groups (list (user-group (name "foo")(id 10)))
					      vlist-null
					      (list (group-entry
						     (name "foo")  (gid 20))))
$2 = (#<<group-entry> name: "foo" password: #f gid: 10 members: ()>)
--8<---------------cut here---------------end--------------->8---

That’s a valid policy (declaration prevails over state), but it does
mean that we can’t really apply the above patch.

(Or we could use much lower UID/GID numbers, which are less likely to be
taken…)

Thoughts?

Ludo’.