defmodule Concat do
  def join(a, b, sep \\ "")

	def join(a, b, _sep) when b == "" do
    a
	end

	def join(a, b, sep) do
	  a <> sep <> b
	end
end

IO.puts(Concat.join("Hello", ""))
IO.puts(Concat.join("Hello", "world", "-"))
IO.puts(is_atom(String))