simp only [Holds]
D : Type
I : Interpretation D
τ : PredName → PredName
head✝ : Definition
tail✝ : List Definition
tail_ih✝ :
  ∀ (V : VarAssignment D) (F : Formula),
    Holds D I V tail✝ (sub τ F) ↔
      Holds D
        { nonempty := ⋯, pred_const_ := I.pred_const_,
          pred_var_ := fun P ds => if ds = [] then Holds D I V tail✝ (pred_var_ (τ P) []) else I.pred_var_ P ds }
        V tail✝ F
x : VarName
phi : Formula
V : VarAssignment D
phi_ih :
  ∀ (V : VarAssignment D),
    Holds D I V (head✝ :: tail✝) (sub τ phi) ↔
      Holds D
        { nonempty := ⋯, pred_const_ := I.pred_const_,
          pred_var_ := fun P ds => if ds = [] then I.pred_var_ (τ P) (List.map V []) else I.pred_var_ P ds }
        V (head✝ :: tail✝) phi
⊢ Holds D I V (head✝ :: tail✝) (exists_ x (sub τ phi)) ↔
    Holds D
      { nonempty := ⋯, pred_const_ := I.pred_const_,
        pred_var_ := fun P ds =>
          if ds = [] then Holds D I V (head✝ :: tail✝) (pred_var_ (τ P) []) else I.pred_var_ P ds }
      V (head✝ :: tail✝) (exists_ x phi)
D : Type
I : Interpretation D
τ : PredName → PredName
head✝ : Definition
tail✝ : List Definition
tail_ih✝ :
  ∀ (V : VarAssignment D) (F : Formula),
    Holds D I V tail✝ (sub τ F) ↔
      Holds D
        { nonempty := ⋯, pred_const_ := I.pred_const_,
          pred_var_ := fun P ds => if ds = [] then Holds D I V tail✝ (pred_var_ (τ P) []) else I.pred_var_ P ds }
        V tail✝ F
x : VarName
phi : Formula
V : VarAssignment D
phi_ih :
  ∀ (V : VarAssignment D),
    Holds D I V (head✝ :: tail✝) (sub τ phi) ↔
      Holds D
        { nonempty := ⋯, pred_const_ := I.pred_const_,
          pred_var_ := fun P ds => if ds = [] then I.pred_var_ (τ P) (List.map V []) else I.pred_var_ P ds }
        V (head✝ :: tail✝) phi
⊢ (∃ d, Holds D I (Function.updateITE V x d) (head✝ :: tail✝) (sub τ phi)) ↔
    ∃ d,
      Holds D
        { nonempty := ⋯, pred_const_ := I.pred_const_,
          pred_var_ := fun P ds => if ds = [] then I.pred_var_ (τ P) (List.map V []) else I.pred_var_ P ds }
        (Function.updateITE V x d) (head✝ :: tail✝) phi