apply Quotient.sound
case h
R : Type
inst✝² : Ring R
inst✝¹ : DecidableEq R
X : Type
inst✝ : DecidableEq X
x₁ x₂ : R[X]
s₁ s₂ : FormalSum R X
⊢ Quotient.mk (formalSumSetoid R X) s₁ + Quotient.mk (formalSumSetoid R X) s₂ =
    Quotient.mk (formalSumSetoid R X) s₂ + Quotient.mk (formalSumSetoid R X) s₁
case h.a
R : Type
inst✝² : Ring R
inst✝¹ : DecidableEq R
X : Type
inst✝ : DecidableEq X
x₁ x₂ : R[X]
s₁ s₂ : FormalSum R X
⊢ s₁ ++ s₂ ≈ s₂ ++ s₁