conv =>
  lhs
  rw [add_assoc]
  rw [← add_assoc]
case cons.h
R : Type
inst✝² : Ring R
inst✝¹ : DecidableEq R
X : Type
inst✝ : DecidableEq X
a b : R
h : R × X
t : List (R × X)
ih : scmul a t ++ scmul b t ≈ scmul (a + b) t
x₀ : X
il₁ : coords (scmul a t) x₀ + coords (scmul b t) x₀ = coords (scmul (a + b) t) x₀
⊢ monomCoeff R X x₀ (a * h.1, h.2) +
      (monomCoeff R X x₀ (b * h.1, h.2) + coords (scmul a t) x₀ + coords (scmul b t) x₀) =
    monomCoeff R X x₀ (a * h.1, h.2) + monomCoeff R X x₀ (b * h.1, h.2) +
      (coords (scmul a t) x₀ + coords (scmul b t) x₀)
no goals