# File lib/sass/script/list.rb, line 42
    def to_s(opts = {})
      raise Sass::SyntaxError.new("() isn't a valid CSS value.") if value.empty?
      return value.reject {|e| e.is_a?(List) && e.value.empty?}.map {|e| e.to_s(opts)}.join(sep_str)
    end