This version of SAT4J adds explanation support in case of unsatisfiability based on a simple version of the QuickXplain algorithm.
Update: the archive was updated on Feb 28 to improve the explanation running time.
As a consequence of the explanation support, the API has changed between 2.0 and 2.1.
Some cleanup has been done: some unused features have been removed (e.g. binary and ternary clauses dedicated data structures).