My Lucid killed my GnomeShell

Version conflict? Dependencies broken? Or just too much like hard work? Whatever the reason, when I ran a dist-upgrade on the Pentium-4 test box recently, Lucid Lynx 10.04 spontaneously decided of its own accord to remove GnomeShell ! Granted it's not my favourite interface , deeply flawed in the current version, but for Lucid to take it out on it's own initiative?! Question: just how does dist-upgrade decide what to keep and what to throw away? If I hadn't examined the list of packages affected, I wouldn't have known a thing until I tried to run GnomeShell next. Would the dist-upgrade have removed GnomeShell while it was running? This doesn't smell right. RC