# File lib/sass/script/functions.rb, line 1916
    def join(list1, list2, separator = identifier("auto"))
      assert_type separator, :String, :separator
      unless %w(auto space comma).include?(separator.value)
        raise ArgumentError.new("Separator name must be space, comma, or auto")
      end
      sep = if separator.value == 'auto'
              list1.separator || list2.separator || :space
            else
              separator.value.to_sym
            end
      list(list1.to_a + list2.to_a, sep)
    end