Elixir function that flattens an array of integers without using List.flatten

flatten.ex

defmodule M do
  def flatten(l) do
    flatten(l, [])
    |> Enum.reverse
  end

  def flatten([], acc) do
    acc
  end
  def flatten([head | tail], acc) when is_integer(head) do
    flatten(tail, [head | acc])
  end
  def flatten([head | tail], acc) when is_list(head) do
    flatten(tail, flatten(head, acc))
  end
end

# tests
[1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 20] =
  M.flatten([1, 2, 3, [4], 5, [6, 7], [[8, [9, 10], [11, [12], [[13, [14]]]]]], 20])