intro x₂ x₃
case h
R : Type
inst✝² : Ring R
inst✝¹ : DecidableEq R
X : Type
inst✝ : DecidableEq X
s₁ : FormalSum R X
x₂ x₃ : R[X]
⊢ ∀ (a b : FormalSum R X),
    Quotient.mk (formalSumSetoid R X) s₁ + Quotient.mk (formalSumSetoid R X) a + Quotient.mk (formalSumSetoid R X) b =
      Quotient.mk (formalSumSetoid R X) s₁ + (Quotient.mk (formalSumSetoid R X) a + Quotient.mk (formalSumSetoid R X) b)
case h
R : Type
inst✝² : Ring R
inst✝¹ : DecidableEq R
X : Type
inst✝ : DecidableEq X
s₁ : FormalSum R X
x₂✝ x₃✝ : R[X]
x₂ x₃ : FormalSum R X
⊢ Quotient.mk (formalSumSetoid R X) s₁ + Quotient.mk (formalSumSetoid R X) x₂ + Quotient.mk (formalSumSetoid R X) x₃ =
    Quotient.mk (formalSumSetoid R X) s₁ + (Quotient.mk (formalSumSetoid R X) x₂ + Quotient.mk (formalSumSetoid R X) x₃)