code: func Sync(mu *sync.Mutex, fs FileSystem) FileSystem {
  	return &syncFS{mu, fs}
  }
docs: // Sync creates a new file system wrapper around fs that locks a mutex
  // during its operations.
  //
  // The contents of files must be immutable, since it has no way of
  // synchronizing access to the ReadSeekCloser from Open after Open
  // returns.