simp only [Formula.lc_at]
phi : Formula
phi_ih :
  ∀ (j : ℕ) (zs : List String),
    Formula.lc_at j (Formula.openList j (List.map free_ zs) phi) ↔ Formula.lc_at (j + zs.length) phi
j : ℕ
zs : List String
⊢ Formula.lc_at j (Formula.openList j (List.map free_ zs) phi).not_ ↔ Formula.lc_at (j + zs.length) phi.not_
phi : Formula
phi_ih :
  ∀ (j : ℕ) (zs : List String),
    Formula.lc_at j (Formula.openList j (List.map free_ zs) phi) ↔ Formula.lc_at (j + zs.length) phi
j : ℕ
zs : List String
⊢ Formula.lc_at j (Formula.openList j (List.map free_ zs) phi) ↔ Formula.lc_at (j + zs.length) phi