# File lib/sass/selector/comma_sequence.rb, line 54
      def do_extend(extends, parent_directives)
        CommaSequence.new(members.map do |seq|
            extended = seq.do_extend(extends, parent_directives)
            # First Law of Extend: the result of extending a selector should
            # always contain the base selector.
            #
            # See https://github.com/nex3/sass/issues/324.
            extended.unshift seq unless seq.has_placeholder? || extended.include?(seq)
            extended
          end.flatten)
      end