unfold sub
σ : Instantiation
τ : MetaInstantiation
x : VarName
phi : Formula
phi_ih : phi.metaVarSet = ∅ → (sub σ τ phi).metaVarSet = ∅
h1 : phi.metaVarSet = ∅
⊢ (sub σ τ (forall_ x phi)).metaVarSet = ∅
σ : Instantiation
τ : MetaInstantiation
x : VarName
phi : Formula
phi_ih : phi.metaVarSet = ∅ → (sub σ τ phi).metaVarSet = ∅
h1 : phi.metaVarSet = ∅
⊢ (forall_ (↑σ x) (sub σ τ phi)).metaVarSet = ∅