Confluent relations
This MR adds:
- The symmetric and reflexive/transitive/symmetric closure of a relation
- Different notions of confluence: diamond property, confluence, local confluence (following the naming conventions in https://www21.in.tum.de/~nipkow/TRaAT/)
- Standard properties such as diamond ⊢ confluence ⊢ local confluence, the Church-Rosser property, Newman's lemma
As part of this MR, I removed the hint database ars
. The name made no sense and it was not used anywhere to my knowledge. If it was used anywhere, it would be very unreliable as it contained hints like rtc_trans
that would generally lead to loops.