code: def _list_existing(filesystem, glob, paths):
      
      globs = _constrain_glob(glob, paths)
      time_start = time.time()
      listing = []
      for g in sorted(globs):
          logger.debug('Listing %s', g)
          if filesystem.exists(g):
              listing.extend(filesystem.listdir(g))
      logger.debug('%d %s listings took %f s to return %d items',
                   len(globs), filesystem.__class__.__name__, time.time() - time_start, len(listing))
      return set(listing)
docs: Get all the paths that do in fact exist. Returns a set of all existing paths.
  
      Takes a luigi.target.FileSystem object, a str which represents a glob and
      a list of strings representing paths.