simp only [IsValid]
F : Formula
σ : VarName → VarName
h1 : Function.Injective σ
h2 : ∀ (D : Type) (I : Interpretation D) (V : VarAssignment D) (E : Env), Holds D I V E F
⊢ (replaceAll σ F).IsValid
F : Formula
σ : VarName → VarName
h1 : Function.Injective σ
h2 : ∀ (D : Type) (I : Interpretation D) (V : VarAssignment D) (E : Env), Holds D I V E F
⊢ ∀ (D : Type) (I : Interpretation D) (V : VarAssignment D) (E : Env), Holds D I V E (replaceAll σ F)