When using resolution, if the empty set {} is derived from a formula like {x,y} {x,y}, does that mean the formula is unsatisfiable? If this is the case, why is {x,y},{x,y},{x,y} satisfiable, and {x},{x},{x,y,z,w} is not satisfiable? 
Resolution and what it means to derive the empty set
algorithms;logic;satisfiability;propositional logic