TCO and append-less List.flatten

flatten.ex

defmodule ListOp do
  def flatten([head | tail]), do: flatten(tail, [head])
  def flatten([], acc), do: Enum.reverse(acc)
  def flatten([ [head | []] | tail], acc), do: flatten(tail, [head | acc])
  def flatten([ [head | rest] | tail], acc), do: flatten(flatten([rest | tail], [head | acc]))
  def flatten([head | tail], acc), do: flatten(tail, [head | acc])
end