A flag DEFS_CHAIN_HISTORY can be used to improve the error message
in case a cycle has been detected. If it is switched off and a cycle has been
detected, the user is notified that there is such a flag.
By chapter:
chapters 1 and 2, maybe 3: Peter White
chapters 6, 7 and 8: David von Oheim
chapter 6, maybe 8: Gerwin Klein
chapters 3, maybe 4: Tanja Vos
chapters 1-3 at least: Stefano Bistarelli