∀ {ι : Type u_1} {A : Type u_2} [inst : DecidableEq ι] [inst_1 : AddMonoid ι] [inst_2 : Semiring A]
  [inst_3 : (m : A) → Decidable (m ≠ 0)],
  Function.RightInverse addMonoidAlgebraRingEquivDirectSum.invFun addMonoidAlgebraRingEquivDirectSum.toFun