Strong compression does not respect embeddings between stories

[feret]

There is currently no test that the result of the strong compression embeds in the initial causal trace. (see the example distributive.ka in test_suites/cflows/distributive)

I think this can be avoided nicely, by restricting agent substitution by allowing the replacement of an agent x, either by itself, or by an agent in the causal past of the event. (It would also reduce the search space, which is good).