simp only [IsValid] at h2
F : Formula
τ : PredName → ℕ → List VarName × Formula
h1 : admits τ F
h2 : F.IsValid
⊢ (replace τ F).IsValid
F : Formula
τ : PredName → ℕ → List VarName × Formula
h1 : admits τ F
h2 : ∀ (D : Type) (I : Interpretation D) (V : VarAssignment D) (E : Env), Holds D I V E F
⊢ (replace τ F).IsValid