unfold Formula.metaVarSet
x : VarName
phi : Formula
phi_ih : ∀ (vs : List VarName), NoMetaVarAndAllFreeInList vs phi → phi.metaVarSet = ∅
vs : List VarName
h1 : NoMetaVarAndAllFreeInList (x :: vs) phi
⊢ (forall_ x phi).metaVarSet = ∅
x : VarName
phi : Formula
phi_ih : ∀ (vs : List VarName), NoMetaVarAndAllFreeInList vs phi → phi.metaVarSet = ∅
vs : List VarName
h1 : NoMetaVarAndAllFreeInList (x :: vs) phi
⊢ phi.metaVarSet = ∅