code: func contains(fs *flag.FlagSet, f *flag.Flag) bool {
  	if fs == nil {
  		return false
  	}
  
  	var in bool
  	fs.VisitAll(func(hf *flag.Flag) {
  		in = in || f.Name == hf.Name
  	})
  	return in
  }
docs: // contains returns true if the given flag is contained in the given flag
  // set or false otherwise.