code: func (nfs *NamedFlagSets) FlagSet(name string) *pflag.FlagSet {
  	if nfs.FlagSets == nil {
  		nfs.FlagSets = map[string]*pflag.FlagSet{}
  	}
  	if _, ok := nfs.FlagSets[name]; !ok {
  		nfs.FlagSets[name] = pflag.NewFlagSet(name, pflag.ExitOnError)
  		nfs.Order = append(nfs.Order, name)
  	}
  	return nfs.FlagSets[name]
  }
docs: // FlagSet returns the flag set with the given name and adds it to the
  // ordered name list if it is not in there yet.