∀x.(A(x)→B(x))
     ----------------------
           A(x)→B(x)              [A(x)]1
      --------------------------------------
                       B(x)                     [¬B(x)]2
                     ---------------------------------------
                                  ⊥
                             --------------(1)
                                 ¬A(x)                            ∃x.¬B(x)
                               ---------------------------------------------- (2)
                                                    ¬A(x)
                                                ----------------
                                                   ∃x.¬A(x)