code: def _expand_url(short_link, subreddit=None):
          
          # Some URL structures for notes
          message_scheme = 'https://reddit.com/message/messages/{}'
          comment_scheme = 'https://reddit.com/r/{}/comments/{}/-/{}'
          post_scheme = 'https://reddit.com/r/{}/comments/{}/'
  
          if short_link == '':
              return None
          else:
              parts = short_link.split(',')
  
              if parts[0] == 'm':
                  return message_scheme.format(parts[1])
              if parts[0] == 'l' and subreddit:
                  if len(parts) > 2:
                      return comment_scheme.format(subreddit, parts[1], parts[2])
                  else:
                      return post_scheme.format(subreddit, parts[1])
              elif not subreddit:
                  raise ValueError('Subreddit name must be provided')
              else:
                  return None
docs: Convert a usernote's URL short-hand into a full reddit URL.
  
          Arguments:
              subreddit: the subreddit the URL is for (PRAW Subreddit object or str)
              short_link: the compressed link from a usernote (str)
  
          Returns a String of the full URL.