code: func parseWithSchedule(expr string, randSeed uint64) (*Schedule, error) {
  	tokens := strings.SplitN(expr, " ", 3)
  	if len(tokens) != 3 || tokens[0] != "with" || tokens[2] != "interval" {
  		return nil, errors.New("expecting format \"with <duration> interval\"")
  	}
  	interval, err := time.ParseDuration(tokens[1])
  	if err != nil {
  		return nil, fmt.Errorf("bad duration %q - %s", tokens[1], err)
  	}
  	if interval < 0 {
  		return nil, fmt.Errorf("bad interval %q - it must be positive", tokens[1])
  	}
  	return &Schedule{interval: interval}, nil
  }
docs: // parseWithSchedule parses "with <interval> interval" schedule string.