code: func SubcommandContextWithParams(params CommandParams) *subcommands.Command {
  	// By default request all scopes used by authctx.Context.
  	params.AuthOptions.Scopes = []string{
  		"https://www.googleapis.com/auth/cloud-platform",
  		"https://www.googleapis.com/auth/firebase",
  		"https://www.googleapis.com/auth/gerritcodereview",
  		"https://www.googleapis.com/auth/userinfo.email",
  	}
  	return &subcommands.Command{
  		Advanced:  params.Advanced,
  		UsageLine: fmt.Sprintf("%s [flags] [--] <bin> [args]", params.Name),
  		ShortDesc: "sets up new LUCI local auth context and launches a process in it",
  		LongDesc:  "Starts local RPC auth server, prepares LUCI_CONTEXT, launches a process in this environment.",
  		CommandRun: func() subcommands.CommandRun {
  			c := &contextRun{}
  			c.params = &params
  			c.registerBaseFlags()
  			c.Flags.StringVar(
  				&c.actAs, "act-as-service-account", "",
  				"Act as a given service account (caller must have iam.serviceAccountActor role).")
  			c.Flags.BoolVar(&c.verbose, "verbose", false, "More logging")
  			return c
  		},
  	}
  }
docs: // SubcommandContextWithParams returns subcommand.Command that can be used to
  // setup new LUCI authentication context for a process tree.