rw [Occurs] at h
case neg
α : Type
as bs cs : List α
a b✝ c : α
A✝ B : PropForm
τ : String → Bool
v : String
b : Bool
A : PropForm
ihA : ¬Occurs v A → eval (fun w => if v = w then b else τ w) A = eval τ A
h : ¬Occurs v (neg A)
⊢ (!eval (fun w => if v = w then b else τ w) A) = !eval τ A
case neg
α : Type
as bs cs : List α
a b✝ c : α
A✝ B : PropForm
τ : String → Bool
v : String
b : Bool
A : PropForm
ihA : ¬Occurs v A → eval (fun w => if v = w then b else τ w) A = eval τ A
h : ¬Occurs v A
⊢ (!eval (fun w => if v = w then b else τ w) A) = !eval τ A