code: func (m *PathRecorderMux) UnlistedHandle(path string, handler http.Handler) {
  	m.lock.Lock()
  	defer m.lock.Unlock()
  	m.trackCallers(path)
  
  	m.pathToHandler[path] = handler
  	m.refreshMuxLocked()
  }
docs: // UnlistedHandle registers the handler for the given pattern, but doesn't list it.
  // If a handler already exists for pattern, Handle panics.