code: func (f *Field) addBSIGroup(bsig *bsiGroup) error {
  	if err := bsig.validate(); err != nil {
  		return errors.Wrap(err, "validating bsigroup")
  	} else if f.hasBSIGroup(bsig.Name) {
  		return ErrBSIGroupExists
  	}
  
  	// Add bsiGroup to list.
  	f.bsiGroups = append(f.bsiGroups, bsig)
  
  	// Sort bsiGroups by name.
  	sort.Slice(f.bsiGroups, func(i, j int) bool {
  		return f.bsiGroups[i].Name < f.bsiGroups[j].Name
  	})
  
  	return nil
  }
docs: // addBSIGroup adds a single bsiGroup to bsiGroups.