∀ (k : Type u_8) (G : Type u_9) [inst : Semiring k] [inst_1 : Mul G],
  MonoidAlgebra.toAdditive k G =
    { toFun := fun x => MonoidAlgebra.mapDomain (⇑Additive.ofMul) x,
      invFun := fun x => AddMonoidAlgebra.mapDomain (⇑Additive.toMul) x, left_inv := ⋯, right_inv := ⋯, map_mul' := ⋯,
      map_add' := ⋯ }