Class 18 : \Sigma is consistent => \Sigma is satisfiable. 
	 -- Construction of MCS from a consistent set 
Class 19 : 
	 -- proof of satisfiability of Maximal consistent set construted in the previous class. 
            Thus completing the proof of consistency implies satisfiability  
         -- Completeness Theorem for PL
	 -- compactness theorem using completeness
         -- maximal satisfiability Iff maximal consistency