simp only [predVarSet] at h1
τ : PredName → PredName
x : VarName
phi : Formula
phi_ih : phi.predVarSet = ∅ → sub τ phi = phi
h1 : (exists_ x phi).predVarSet = ∅
⊢ sub τ (exists_ x phi) = exists_ x phi
τ : PredName → PredName
x : VarName
phi : Formula
phi_ih : phi.predVarSet = ∅ → sub τ phi = phi
h1 : phi.predVarSet = ∅
⊢ sub τ (exists_ x phi) = exists_ x phi