code: func (e MultiError) Add(err error) MultiError {
  	if err == nil {
  		return e
  	}
  	me := e
  	if me.err == nil {
  		me.err = err
  		return me
  	}
  	me.errors = append(me.errors, me.err)
  	me.err = err
  	return me
  }
docs: // Add adds an error returns a new MultiError object.