Class 12 : Normal Forms 
-- literals, positive literals, negative literals. 
-- CNF, DNF  
-- existence of  equivalent formulae in CNF and DNF for a given formula using truth tables 
-- time complexity of SAT -- NP complete
-- quest for some fragment which has polytime complexity for SAT
-- Horn formulae : CNF with each clause having at most one literal
-- Algorithm for checking satisfiability of Horn Formulae