P ⊢ (¬ P) → Q 



            P   [¬ P]1
          ----------- (¬ E)
              ⊥
           -------- (⊥)
              Q
         ------------ (→ I)(1)
         (¬ P) → Q



P, Q ⊢ ¬ (P → ¬ Q) 



            P   [P → ¬ Q]1
          ---------------- (→ E)
      Q         ¬ Q
    ----------------- (⊥)
            ⊥
       ------------ (¬  I)(1)
       ¬ (P → ¬ Q)