I read this
Note that some index types may require additional attributes to be specified; this does not invalidate them for the context of this question. Only index types that include illegal attribute combinations or non existing attributes should be discarded. Also note that this question is not about the exact spelling of the various options
and immediately realized that it was another question by Hugo Kornelis.
So back to BOL, Google and multiple cups of coffee.
But once again I learned something ... Mr Kornelis I thank you.