On the redundancy of short and heterogeneous sequences of belief revisions

Forgetting a specific belief revision episode may not erase information because the other revisions may provide the same information or allow to deduce it. Whether it does was proved coNP-hard for sequence of two arbitrary lexicographic revision or arbitrarily long lexicographic Horn revision. A polynomial algorithm is presented for the case of two Horn revision. Heterogeneous sequences of revisions were proved to belong in Delta2. Their previously proved coNP-hardness is enhanced by a proof of NP-hardness.