code: func RegisterDefaultAuth(a Authenticator) {
  	if a == nil {
  		panic("a is nil")
  	}
  	defaultAuth.Lock()
  	defer defaultAuth.Unlock()
  	if defaultAuth.Authenticator != nil {
  		panic("default prpc authenticator is already set")
  	}
  	defaultAuth.Authenticator = a
  }
docs: // RegisterDefaultAuth sets a default authenticator that is used unless
  // Server.Authenticator is provided.
  //
  // It is supposed to be used during init() time, to configure the default
  // authentication method used by the program.
  //
  // For example, see luci-go/appengine/gaeauth/server/prpc.go, that gets imported
  // by LUCI GAE apps. It sets up authentication based on LUCI auth framework.
  //
  // Panics if 'a' is nil or a default authenticator is already set.