I've been reading the old mail from this mailing list, and I've
noticed some discussion on the definition of what is a single twist.
As far as I can see, the definition of a twist should be the change in
position of all the cubies in a single plane performed by a single
motion. In other words that should include a 90 degree twist, a 180
degree twist, and a twist of the middle slice (the last one can be
thought of as holding the U and D parts of the cube in place with one
hand, while moving the middle slice with the other).