You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: main.go
    path of file: ./repos/nats.go/examples/nats-pub
    
    the code of the file until where you have to start completion: // Copyright 2012-2022 The NATS Authors
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
    
func main() {
	var urls = flag.String("s", nats.DefaultURL, "The nats server URLs (separated by comma)")
	var userCreds = flag.String("creds", "", "User Credentials File")
	var nkeyFile = flag.String("nkey", "", "NKey Seed File")
	var tlsClientCert = flag.String("tlscert", "", "TLS client certificate file")
	var tlsClientKey = flag.String("tlskey", "", "Private key file for client certificate")
	var tlsCACert = flag.String("tlscacert", "", "CA certificate to verify peer against")
	var reply = flag.String("reply", "", "Sets a specific reply subject")
	var showHelp = flag.Bool("h", false, "Show help message")

	log.SetFlags(0)
	flag.Usage = usage
	flag.Parse()

	if *showHelp {
		showUsageAndExit(0)
	}

	args := flag.Args()
	if len(args) != 2 {
		showUsageAndExit(1)
	}

	// Connect Options.
	opts := []nats.Option{nats.Name("NATS Sample Publisher")}

	if *userCreds != "" && *nkeyFile != "" {
		log.Fatal("specify -seed or -creds")
	}

	// Use UserCredentials
	if *userCreds != "" {
		opts = append(opts, nats.UserCredentials(*userCreds))
	}

	// Use TLS client authentication
	if *tlsClientCert != "" && *tlsClientKey != "" {
		opts = append(opts, nats.ClientCert(*tlsClientCert, *tlsClientKey))
	}

	// Use specific CA certificate
	if *tlsCACert != "" {
		opts = append(opts, nats.RootCAs(*tlsCACert))
	}

	// Use Nkey authentication.
	if *nkeyFile != "" {
		opt, err := nats.NkeyOptionFromSeed(*nkeyFile)
		if err != nil {
			log.Fatal(err)
		}
		opts = append(opts, opt)
	}

	// Connect to NATS
	nc, err := nats.Connect(*urls, opts...)
	if err != nil {
		log.Fatal(err)
	}
	defer nc.Close()

	subj, msg := args[0], []byte(args[1])

	if reply != nil && *reply != "" {
		nc.PublishRequest(subj, *reply, msg)
	} else {
		nc.Publish(subj, msg)
	}

	nc.Flush()

	if err := nc.LastError(); err != nil {
		log.Fatal(err)
	} else {
		log.Printf("Published [%s] : '%s'\n", subj, msg)
	}
}