apply add_assoc_aux
case a
R : Type
inst✝² : Ring R
inst✝¹ : DecidableEq R
X : Type
inst✝ : DecidableEq X
x₁✝ x₂ x₃ : R[X]
x₁ : FormalSum R X
⊢ Quotient.mk (formalSumSetoid R X) x₁ + x₂ + x₃ = Quotient.mk (formalSumSetoid R X) x₁ + (x₂ + x₃)
no goals