obtain rfl := show f = fun i : Fin _ => f' i by subst h; simp
x : BoolExpr Nat
f : Fin x.vars → Bool
f' : Nat → Bool
h : (fun i => if h : i < x.vars then f ⟨i, h⟩ else false) = f'
⊢ eval f x.attach = eval f' x
x : BoolExpr Nat
f' : Nat → Bool
h : (fun i => if h : i < x.vars then (fun i => f' ↑i) ⟨i, h⟩ else false) = f'
⊢ eval (fun i => f' ↑i) x.attach = eval f' x