docs: def root_urns_for_deletion(self):
      """Roots of the graph of urns marked for deletion."""
      roots = set()
      for urn in self._urns_for_deletion:
        new_root = True
  
        str_urn = utils.SmartUnicode(urn)
        fake_roots = []
        for root in roots:
          str_root = utils.SmartUnicode(root)
  
          if str_urn.startswith(str_root):
            new_root = False
            break
          elif str_root.startswith(str_urn):
            fake_roots.append(root)
  
        if new_root:
          roots -= set(fake_roots)
          roots.add(urn)
  
      return roots