code: func (es *MultiError) Add(err error) {
  	if err == nil {
  		return
  	}
  	if merr, ok := err.(MultiError); ok {
  		*es = append(*es, merr...)
  	} else {
  		*es = append(*es, err)
  	}
  }
docs: // Add adds the error to the error list if it is not nil.