code: func (d Duration) Sub(v Duration) (Duration, error) {
  	if &v == nil {
  		return d, nil
  	}
  	dsum, err := SubInt64(int64(d.Duration), int64(v.Duration))
  	if err != nil {
  		return Duration{}, errors.Trace(err)
  	}
  	if d.Fsp >= v.Fsp {
  		return Duration{Duration: gotime.Duration(dsum), Fsp: d.Fsp}, nil
  	}
  	return Duration{Duration: gotime.Duration(dsum), Fsp: v.Fsp}, nil
  }
docs: // Sub subtracts d to d, returns a duration value.