apply @Quotient.ind₂ (motive := fun x₂ x₃ : R[X] => (⟦s₁⟧ + x₂) + x₃ = ⟦s₁⟧ + (x₂ + x₃))
R : Type
inst✝² : Ring R
inst✝¹ : DecidableEq R
X : Type
inst✝ : DecidableEq X
s₁ : FormalSum R X
x₂ x₃ : R[X]
⊢ Quotient.mk (formalSumSetoid R X) s₁ + x₂ + x₃ = Quotient.mk (formalSumSetoid R X) s₁ + (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)