apply @Quotient.ind₂ (motive := fun x₁ x₂ : R[X] => x₁ + x₂ = x₂ + x₁)
R : Type
inst✝² : Ring R
inst✝¹ : DecidableEq R
X : Type
inst✝ : DecidableEq X
x₁ x₂ : R[X]
⊢ x₁ + x₂ = x₂ + x₁
case h
R : Type
inst✝² : Ring R
inst✝¹ : DecidableEq R
X : Type
inst✝ : DecidableEq X
x₁ x₂ : R[X]
⊢ ∀ (a b : FormalSum R X),
    Quotient.mk (formalSumSetoid R X) a + Quotient.mk (formalSumSetoid R X) b =
      Quotient.mk (formalSumSetoid R X) b + Quotient.mk (formalSumSetoid R X) a