Resolution
Examples 
Soundness of resolution rule.

Res*(F) 

Proposition 1: 
If \emptyset \in Res*(F) then F is unsatisfiable. 
Proof : (by contradiction and using soundness of resolution rule) 

Proposition 2: 
If  F is unsatisfiable  then  \emptyset \in Res*(F)  
Proof : (by induction on #prop_variables appearing in F
Base case : when F has only one variable. 


