cases 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.predVarSet = ∅ ∧ psi.predVarSet = ∅
⊢ sub τ (phi.iff_ psi) = phi.iff_ psi
case intro
τ : PredName → PredName
phi psi : Formula
phi_ih : phi.predVarSet = ∅ → sub τ phi = phi
psi_ih : psi.predVarSet = ∅ → sub τ psi = psi
left✝ : phi.predVarSet = ∅
right✝ : psi.predVarSet = ∅
⊢ sub τ (phi.iff_ psi) = phi.iff_ psi