code: func RegisterHandler(code int, f http.Handler) {
  	handlerMu.Lock()
  	defer handlerMu.Unlock()
  	switch f {
  	case nil:
  		delete(handlerMap, code)
  	default:
  		handlerMap[code] = f
  	}
  }
docs: // RegisterHandler registers the given HandlerFunc to serve HTTP requests for
  // the given status code. Use CtxErr and CtxDomain to retrieve extra values set
  // on the request in f (if any).
  //
  // Despite registering the handler for the code, f is responsible for calling
  // WriteHeader(code) since it may want to set response headers first.
  //
  // To delete a Handler, call RegisterHandler with nil for the second argument.