simp only [Holds]
D : Type
I : Interpretation D
M : MetaValuation D
head✝ : Definition
tail✝ : List Definition
tail_ih✝ :
  ∀ (V V' : Valuation D) (F : Formula) (vs : List VarName),
    NoMetaVarAndAllFreeInList vs F → (∀ v ∈ vs, V v = V' v) → (Holds D I V M tail✝ F ↔ Holds D I V' M tail✝ F)
phi psi : Formula
phi_ih :
  ∀ (V V' : Valuation D) (vs : List VarName),
    NoMetaVarAndAllFreeInList vs phi →
      (∀ v ∈ vs, V v = V' v) → (Holds D I V M (head✝ :: tail✝) phi ↔ Holds D I V' M (head✝ :: tail✝) phi)
psi_ih :
  ∀ (V V' : Valuation D) (vs : List VarName),
    NoMetaVarAndAllFreeInList vs psi →
      (∀ v ∈ vs, V v = V' v) → (Holds D I V M (head✝ :: tail✝) psi ↔ Holds D I V' M (head✝ :: tail✝) psi)
V V' : Valuation D
vs : List VarName
h2 : ∀ v ∈ vs, V v = V' v
h1 : NoMetaVarAndAllFreeInList vs phi ∧ NoMetaVarAndAllFreeInList vs psi
⊢ Holds D I V M (head✝ :: tail✝) (phi.imp_ psi) ↔ Holds D I V' M (head✝ :: tail✝) (phi.imp_ psi)
D : Type
I : Interpretation D
M : MetaValuation D
head✝ : Definition
tail✝ : List Definition
tail_ih✝ :
  ∀ (V V' : Valuation D) (F : Formula) (vs : List VarName),
    NoMetaVarAndAllFreeInList vs F → (∀ v ∈ vs, V v = V' v) → (Holds D I V M tail✝ F ↔ Holds D I V' M tail✝ F)
phi psi : Formula
phi_ih :
  ∀ (V V' : Valuation D) (vs : List VarName),
    NoMetaVarAndAllFreeInList vs phi →
      (∀ v ∈ vs, V v = V' v) → (Holds D I V M (head✝ :: tail✝) phi ↔ Holds D I V' M (head✝ :: tail✝) phi)
psi_ih :
  ∀ (V V' : Valuation D) (vs : List VarName),
    NoMetaVarAndAllFreeInList vs psi →
      (∀ v ∈ vs, V v = V' v) → (Holds D I V M (head✝ :: tail✝) psi ↔ Holds D I V' M (head✝ :: tail✝) psi)
V V' : Valuation D
vs : List VarName
h2 : ∀ v ∈ vs, V v = V' v
h1 : NoMetaVarAndAllFreeInList vs phi ∧ NoMetaVarAndAllFreeInList vs psi
⊢ Holds D I V M (head✝ :: tail✝) phi → Holds D I V M (head✝ :: tail✝) psi ↔
    Holds D I V' M (head✝ :: tail✝) phi → Holds D I V' M (head✝ :: tail✝) psi