docs: def _process_stream_delta(self, delta_stream):
          """Bookkeeping on internal data structures while iterating a stream."""
          for pchange in delta_stream:
              if pchange.kind == ChangeType.ADD:
                  self.policy_files.setdefault(
                      pchange.file_path, PolicyCollection()).add(pchange.policy)
              elif pchange.kind == ChangeType.REMOVE:
                  self.policy_files[pchange.file_path].remove(pchange.policy)
              elif pchange.kind in (ChangeType.MOVED, ChangeType.MODIFIED):
                  if pchange.policy.file_path != pchange.previous.file_path:
                      self.policy_files[pchange.previous.file_path].remove(pchange.previous)
                      if (pchange.policy.file_path in self.policy_files and
                              pchange.policy.name in self.policy_files[pchange.file_path]):
                          self.policy_files[pchange.file_path][pchange.policy.name] = pchange.policy
                      else:
                          self.policy_files.setdefault(
                              pchange.file_path, PolicyCollection()).add(pchange.policy)
                  else:
                      self.policy_files[pchange.file_path][pchange.policy.name] = pchange.policy
              yield pchange