simp only [predVarSet] at h1
τ : PredName → PredName
phi psi : Formula
phi_ih : phi.predVarSet = ∅ → sub τ phi = phi
psi_ih : psi.predVarSet = ∅ → sub τ psi = psi
h1 : (phi.iff_ psi).predVarSet = ∅
⊢ sub τ (phi.iff_ psi) = phi.iff_ psi
τ : PredName → PredName
phi psi : Formula
phi_ih : phi.predVarSet = ∅ → sub τ phi = phi
psi_ih : psi.predVarSet = ∅ → sub τ psi = psi
h1 : phi.predVarSet ∪ psi.predVarSet = ∅
⊢ sub τ (phi.iff_ psi) = phi.iff_ psi