Semantics of FOL
-- graph theory 
-- \Sigma \entails \alpha
-- Lemma : two assignments agree on FV(\alpha) in a structure L, 
           then  M \entails \alpha under s_1 
 			iff 
                  M \entails \alpha under s_1    
-- Proof of lemma : terms, atomic formuals. 
-- Examples : 
-- L =(R(,))
   M1=(\nat, <)
   M2=(\nat, divides)
   M3=(pow(nat), \subset)
   formulas : R(x,y) 
            : \exists x \forall y (R(x,y)).