Exercise 3: A Lighter Fix for a Wrong-Distro-Booting Dual-Linux System — Possible Solution
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-- The lighter fix worth trying first --
--
-- Per this chapter's own tip-box, for exactly this scenario -- the
-- wrong distro's own menu becoming authoritative after an update, a
-- direct instance of grub1-7's own "competing package managers"
-- problem -- running just os-prober plus update-grub inside a chroot
-- into the DESIRED authoritative distro is often sufficient on its
-- own, without needing a full grub-install. Concretely: boot a live
-- USB, mount and chroot into whichever distro should actually be
-- authoritative, and simply run update-grub there (which itself
-- invokes os-prober as part of the regeneration process, per
-- grub1-2's own material on 30_os-prober).

-- Why this is likely sufficient here --
--
-- The scenario as described is specifically that the wrong distro's
-- MENU is now showing -- not that GRUB itself has failed to load at
-- all, or that no bootable device can be found. This means GRUB's own
-- installation (its boot sector or EFI entry) is still fundamentally
-- functional; it's just that a DIFFERENT distro's grub.cfg is
-- currently the one being read, likely because that other distro's
-- own package manager reinstalled its GRUB into the boot sector
-- during a routine update, exactly grub1-7's own described failure
-- mode. Since the underlying GRUB installation mechanism itself isn't
-- broken -- only which distro's configuration is currently active --
-- reinstalling GRUB entirely (grub-install) would be solving a
-- problem that doesn't actually exist here. Regenerating the desired
-- distro's own grub.cfg via update-grub, and ensuring THAT distro's
-- GRUB is the one actually controlling the boot sector/EFI entry, is
-- the more targeted, appropriately lighter fix for this specific
-- symptom.

WHY THIS WORKS AS AN ANSWER
------------------------------
This names the chapter's own specific lighter-fix recommendation and
justifies why it fits this exact symptom by distinguishing "wrong
config is active" (this scenario) from "GRUB itself is broken"
(the scenario a full grub-install would actually be needed for).