simp only [Set.union_comm]
Q : Formula
Δ : Set Formula
h1 : IsDeduct Δ Q
⊢ ∀ (Γ : Set Formula), IsDeduct (Γ ∪ Δ) Q
Q : Formula
Δ : Set Formula
h1 : IsDeduct Δ Q
⊢ ∀ (Γ : Set Formula), IsDeduct (Δ ∪ Γ) Q