apply Quotient.sound
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)
case h.a
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
⊢ s₁ ++ x₂ ++ x₃ ≈ s₁ ++ (x₂ ++ x₃)