Meanwhile I have carefully studied your implementation.

There is only a single point I am not certain about:

  As I understand isolation levels, they garantee that some bad
  things will not happen but not that all not bad thing will happen.

  For "read committed" this means: it garantees that I will
  only see committed transactions but not necessarily that I will see
  the effect of a transaction as soon as it is committed.

  Your conflict resolution requires that it sees a transaction as
  soon as it is commited.

  The supported relational databases may have this property -- but
  I expect we do not have a written garantee that this will definitely
  be the case.

I plan to make a test which tries to provoke a conflict resolution
failure -- and gives me confidance that the "read committed" of
Postgres really has the required property.

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