² I learned about keeping backups the hard way -- by accidentially destroying the TeX sources of the Diplomarbeit while performing house-keeping tasks on my trusty old Atari ST. Of course, the only backup was months old, since I had recycled the newer backups for their media.
³ The contents of this report is not very current; the semantic model has been rewritten in the last years.The current, up-to-date, never-to-change-again semantics can be found in my PhD thesis. (Soon to be available in every major university library. I hope.)