| Line 43: | 
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|   |  |   |  | 
|   | ===Step 3 (Optional)===  |   | ===Step 3 (Optional)===  | 
|   | + |  | 
|   | + | Check that <math>A \Rightarrow A</math> is a theorem of classical propositional calculus.  | 
|   |  |   |  | 
|   | <pre>  |   | <pre>  | 
| − | Check that A => A is a theorem of classical propositional calculus.
  |   | 
| − | 
  |   | 
|   |     A   A  |   |     A   A  | 
|   |     o---o       o---o  |   |     o---o       o---o  | 
|   |     |           |  |   |     |           |  | 
|   |     @     =     @     =     @  |   |     @     =     @     =     @  | 
|   | + | </pre>  | 
|   |  |   |  | 
|   | Check.  |   | Check.  | 
| − | </pre>
  |   | 
|   |  |   |  | 
|   | ===Step 4===  |   | ===Step 4===  |