intro s₁ s₂
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
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