COFE distance is no longer trivial at index 0.
This way we avoid many one-off indexes and no longer need special cases for index 0 in many definitions. For example, the definition of the distance relation on option and excl has become much easier. Also, uPreds no longer need to hold at index 0. In order to make this change possible, we had to change the notions of "contractive functions" and "chains" slightly. Thanks to Aleš Bizjak and Amin Timany for suggesting this change and to help with the proofs.