code: func (r *Record) Tag(tag []byte) (v Aux, ok bool) {
  	if len(tag) < 2 {
  		panic("sam: tag too short")
  	}
  	for _, aux := range r.AuxFields {
  		if aux.matches(tag) {
  			return aux, true
  		}
  	}
  	return nil, false
  }
docs: // Tag returns an Aux tag whose tag ID matches the first two bytes of tag and true.
  // If no tag matches, nil and false are returned.