code: func (m *PathRecorderMux) UnlistedHandlePrefix(path string, handler http.Handler) {
  	if !strings.HasSuffix(path, "/") {
  		panic(fmt.Sprintf("%q must end in a trailing slash", path))
  	}
  
  	m.lock.Lock()
  	defer m.lock.Unlock()
  	m.trackCallers(path)
  
  	m.prefixToHandler[path] = handler
  	m.refreshMuxLocked()
  }
docs: // UnlistedHandlePrefix is like UnlistedHandle, but matches for anything under the path.  Like a standard golang trailing slash.