∀ {R : Type u_1} [inst : Semiring R] (a b : R) (m n : ℕ),
  (Polynomial.monomial (m + 1 + (n + 1) - 1)) (a * (b * ↑(m + 1))) +
      (Polynomial.monomial (m + 1 + (n + 1) - 1)) (a * (b * ↑(n + 1))) =
    (Polynomial.monomial (m + 1 - 1 + (n + 1))) (a * (b * ↑(m + 1))) +
      (Polynomial.monomial (m + 1 + (n + 1 - 1))) (a * (b * ↑(n + 1)))