code: func sortHarvest(h reddit.Harvest) []string {
  	things := merge(
  		postsAsThings(h.Posts),
  		commentsAsThings(h.Comments),
  		messagesAsThings(h.Messages),
  	)
  	sort.Sort(byCreationTime{things})
  
  	names := make([]string, len(things))
  	for i, t := range things {
  		names[i] = t.Name()
  	}
  
  	return names
  }
docs: // sortHarvest returns the list of names of Reddit elements in a harvest sorted
  // by creation time to the younger elements appear first in the slice.