Resolution
Soundness of resolution inference rule:
(li li-1 li+1 lk) li
                mj (m1 mj-1 mj+1 ... mn)
(li li-1 li+1 lk) (m1 mj-1 mj+1 ... mn)
where li and mj are complementary literals.
n What if li and mj are false?
n What if li and mj are true?