Hans Kleine Büning Xishun Zhao An Extension of Deficiency and Minimal Unsatisfiability of Quantified Boolean Formulas. 115-123 2007 3 JSAT 3-4 http://jsat.ewi.tudelft.nl/content/volume3/JSAT3_7_KleineBuening.pdf db/journals/jsat/jsat3.html#BuningZ07a