# File lib/sass/script/functions.rb, line 1282
    def append(list, val, 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
      sep = list.separator if list.is_a?(Sass::Script::List)
      Sass::Script::List.new(
        list.to_a + [val],
        if separator.value == 'auto'
          sep || :space
        else
          separator.value.to_sym
        end)
    end