Creating a macro with elixir to vice versa nested math operations

opposite_math_macro.exs

defmodule OppositeMath do
  @moduledoc """
  This module helps with oppositting nested math formuals like 2 + 5 * 4 + 6 => 2 - 5 / 4 - 6 for sake of learning elixir macros
  """

  @doc """
  This macro opposites all math symbols in a formula

  ## Examples
    iex> OppositeMath.opposite 1+1
    0

    iex> OppositeMath.opposite 2 * 2 + 5
    -4.0
  """
  defmacro opposite(operation) do
    build_opposite(operation)
  end

  ###
  # Macro Helpers
  ###

  defp build_opposite({op, kern, [a, b]}) do
    case op do
      :+ -> {:-, kern, [build_opposite(a), build_opposite(b)]}
      :- -> {:+, kern, [build_opposite(a), build_opposite(b)]}
      :* -> {:/, kern, [build_opposite(a), build_opposite(b)]}
      :/ -> {:*, kern, [build_opposite(a), build_opposite(b)]}
    end
  end

  defp build_opposite(arg), do: arg
end