∀ {R : Type u_1} {M : Type u_2} [inst : Semiring R] [inst_1 : Add M] (r : Rᵐᵒᵖ) (x : Mᵃᵒᵖ),
  AddMonoidAlgebra.opRingEquiv.symm (AddMonoidAlgebra.single x r) =
    MulOpposite.op (AddMonoidAlgebra.single (AddOpposite.unop x) (MulOpposite.unop r))