After ten long years, the satisficing results are finally here! :)

Jokes aside, I reimplemented the suggested change and tested it:
https://ai.dmi.unibas.ch/_experiments/ai/downward/issue384/data/issue384-v1-opt-eval/issue384-v1-opt-issue384-base-issue384-v1-compare.html
https://ai.dmi.unibas.ch/_experiments/ai/downward/issue384/data/issue384-v1-sat-eval/issue384-v1-sat-issue384-base-issue384-v1-compare.html

Unfortunately the results are fairly terrible, we lose coverage in all configurations, and in some quite significantly (for example -11 in lama). Most prominently we often find fewer landmarks. For most domains it is a rather small loss, but for example in pathways we only find about half as many landmarks as before. There are a few domains however where we find more, for example in sokoban. The relative loss/gain in landmarks does however not always correspond to faster/slower search time: For example in barman-sat14 we only lose 2 landmarks per task (out of ~70 to ~90 depending on the task), but lm_rhw-v1 cannot solve a single instance  while the base version solved all instances within 110 seconds.

My next step would be to try and look into some domains where the differences are significant (like barman) and try to figure out what "types" of landmarks we lose/gain. Maybe this can give us an idea for a better disjunction class?

Since the results are not good I did not create a pull request but you can see the code changes here, there are very small:
https://github.com/salome-eriksson/downward/compare/main...salome-eriksson:downward:issue384
(If we want to go with variable id as equivalence relation we could probably also remove all the code regarding disjunction classes and directly use the variable id where relevant; but I wanted to keep the code change small and keep the possibility of defining different equivalence relations.)

I removed the "superseder" value (issue289) since the issue at hand is not
superseded.

I implemented this change and tested it on the suite optimal_with_ipc11(). I
added the scatterplots for number of landmarks and search time. Interestingly
enough there are in most cases less disjunctive landmarks over finite domain
variables found then over PDDL predicates.
The results for satsificing will follow...

This is a follow up to issue289.

There Malte wrote:
"After we fix this, I suggest that we also look at different ways of restricting
which disjunctive landmarks to allow rather than looking at predicate names.
Using predicate names always looked like a bit of a hack to me, since the search
component of the planner is otherwise based on the finite-domain representation
of the planner, which has no such thing. I'd like to see what happens if we look
at other criteria instead, e.g. only allowing disjunctive landmarks where all
facts refer to the same finite-domain variable."