Hw5.lean

theorem problem1 (A B : Prop) : (A → B) → (¬ B → ¬ A) := sorry

theorem problem2 (A B : Prop) : ¬ (A ∨ B) ↔ ¬ A ∧ ¬ B := sorry 

theorem problem3 (A B C : Prop) (h : B ↔ C) : (A ∨ B) ↔ (A ∨ C) := sorry

theorem problem4 (A B : Prop) : (A → B) ↔ ¬ A ∨ B := sorry 

theorem problem5 (A B : Prop) : ¬ A → A → B := sorry