Hi, On this page http://docs.python.org/3/library/constants.html?highlight=notimplemented#Not... It says that NotImplemented should be returned by rich comparison special methods. But it doesn't mention using it with the abc module. Yet on this page http://docs.python.org/3/library/abc.html#module-abc It says that __subclasshook__() methods should return NotImplemented. However, on this page http://docs.python.org/3/library/exceptions.html?highlight=notimplemented#No... It says that abstract methods should raise NotImplementedError. Maybe someone could unify the coverage of NotImplemented and NotImplementedError and explain when to use one and when to use the other? -- Mark Summerfield, Qtrac Ltd, www.qtrac.eu C++, Python, Qt, PyQt - training and consultancy "Programming in Go" - ISBN 0321774639 http://www.qtrac.eu/gobook.html