code: func ServeMux(m ServeMuxMap) {
  	if len(os.Args) != 2 {
  		fmt.Fprintf(os.Stderr,
  			"Invoked improperly. This is an internal command that shouldn't\n"+
  				"be manually invoked.\n")
  		os.Exit(1)
  	}
  
  	opts, ok := m[os.Args[1]]
  	if !ok {
  		fmt.Fprintf(os.Stderr, "Unknown plugin: %s\n", os.Args[1])
  		os.Exit(1)
  	}
  
  	Serve(opts)
  }
docs: // ServeMux is like Serve, but serves multiple types of plugins determined
  // by the argument given on the command-line.
  //
  // This command doesn't return until the plugin is done being executed. Any
  // errors are logged or output to stderr.