Entanglement

The entanglement implementation in master is exponential since we store etangled var set (set) information directly on the var. So if you have 9 vars in an eset one call to update-dom will trigger a lot of unnecessary computation.

A better implementation would store entangled var sets elsewhere. Then we have to consider data structure representation. We can't quite do what we do with the constraint store becaues if two vars that belong to two different esets become entangled we need to merge the esets together. However we can make it work so that we never do more than two lookups.

Consider the following:

{a 0 b 0 c 0
 x 1 y 1 z 1}

Where 0 points to the eset #{a b c} and 1 points to the eset #{x y z}. Now imagine that a and x become entangled. We can merge the two esets and add a new entry 2. 0 will now point to 2 and 1 will also point to 2.

We could imagine another eset #{p q r} where the vars point to 3. If p becomes entangled with a then we just need to merge the contents of 2 with 3 and add a new entry 4. We can then delete 2 and the eset stored there and repoint 0 to 4 and 1 to 4 and 3 to 4.

The cost of updates grow with the number of esets that must be merged. One way to prevent merging of esets early on is to have a threshold, say 8. Prior to this threshold we simply update the individual vars.