# File lib/sass/script/functions.rb, line 1308
    def join(list1, list2, separator = Sass::Script::String.new("auto"))
      assert_type separator, :String
      unless %w[auto space comma].include?(separator.value)
        raise ArgumentError.new("Separator name must be space, comma, or auto")
      end
      sep1 = list1.separator if list1.is_a?(Sass::Script::List) && !list1.value.empty?
      sep2 = list2.separator if list2.is_a?(Sass::Script::List) && !list2.value.empty?
      Sass::Script::List.new(
        list1.to_a + list2.to_a,
        if separator.value == 'auto'
          sep1 || sep2 || :space
        else
          separator.value.to_sym
        end)
    end