code: func (s *State) err(err ...error) error {
  	if s.seenErrs == nil {
  		s.seenErrs = stringset.New(len(err))
  	}
  	for _, e := range err {
  		if bt, _ := e.(BacktracableError); bt == nil || s.seenErrs.Add(bt.Backtrace()) {
  			s.errors = append(s.errors, e)
  		}
  	}
  	return s.errors
  }
docs: // err adds errors to the list of errors and returns the list as MultiError,
  // deduplicating errors with identical backtraces.