∀ {ι : Type u_1} [inst : DecidableEq ι] {A : ι → Type u_2} [inst_1 : Add ι] [inst_2 : (i : ι) → AddCommMonoid (A i)]
  [inst_3 : DirectSum.GNonUnitalNonAssocSemiring A] {i j : ι} (a : A i) (b : A j),
  (DirectSum.of A (i + j)) (GradedMonoid.GMul.mul a b) = (DirectSum.of A i) a * (DirectSum.of A j) b