[A]1      [A→B]3 
                --------------------------- (→E)
                            B                             [¬B]2
                     ------------------------------------------------- (¬E)
                                             ⊥      
                                            ----- (¬I)1
                                             ¬A 
                                      ----------------- (→I)2
                                          (¬)B→¬A     
                                   ------------------------ (→I)3
                                        (A→B) → ((¬B)→¬A)