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This better seals off their definition. Although it did not give
much of a speedup, I think it is conceptually nicer.

With Set Printing All, these notations make me loose overview entirely.

Also, make our redefinition of done more robust under different
orders of Importing modules.

Also do some minor clean up.

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.

This way we can more easily state lemmas for concrete languages for
arbitrary global functors.

This reverts commit 5161e463.

This way, they are non-delta unfoldable constants, which showed a positive
impact on the performance of setoid rewriting. We may want to do this for
other cmra/cofe structures too.