defmodule P7 do
  # public function that calls a private one with accumulator
  def flatten(list), do: do_flatten(list, [])
  # base case - empty list
  defp do_flatten([], result), do: result
end

ExUnit.start

defmodule P7Test do
  use ExUnit.Case

  test "P7.flatten" do
    # step 1 - empty list
    assert P7.flatten([]) == []
  end
end