Renato Bruni Antonio Sassano Restoring Satisfiability or Maintaining Unsatisfiability by finding small Unsatisfiable Subformulae. 162-173 2001 9 Electronic Notes in Discrete Mathematics http://dx.doi.org/10.1016/S1571-0653(04)00320-8 db/journals/endm/endm9.html#BruniS01