Class 13 : Horn Formulae 
  
Horn Formulae : CNF with each clause having at most one positive literal. 
Algorithm for satisfiability-greedy algo
Termination proof
Time complexity-polytime
Correctness.
Soundness : If alogH says formula \alpha is satisfiable then \alpha is satisfiable.
Proof idea : algoH gives an assignment, we prove that this satisfies \alpha. 
 
Completeness:  If the given formula \alpha is satisfiable then algoH prints "satisfiable".
Proof idea: Prove the contrapositive of above statement, using the following claim.
 
Claim : if a proposition is marked true then it remains true in all
possible satisfying assignments of the given Horn Formulae

-- Horn formulae satisfying algorithm and Prolog. 

