simp [coords, ← ih, add_assoc]
case cons
R : Type u_1
inst✝² : Ring R
inst✝¹ : DecidableEq R
X : Type u_2
inst✝ : DecidableEq X
s₂ : FormalSum R X
x₀ : X
h : R × X
t : List (R × X)
ih : coords t x₀ + coords s₂ x₀ = coords (t ++ s₂) x₀
⊢ coords (h :: t) x₀ + coords s₂ x₀ = coords (h :: t ++ s₂) x₀
no goals