Elixir noob code to flatten array

flatten_array.exs

defmodule Citrusbyte do
  def flatten([head | tail]) do
    if(is_list(head)) do
      flatten(head) ++ flatten(tail)
    else
      [head] ++ flatten(tail)
    end
  end
  def flatten([]) do
    []
  end
end

nested = [[4, 5, 6, [1, 2, [1, 2, 3], 3]], 1, 2, 3]

IO.inspect Citrusbyte.flatten(nested)