code: func (l *Locking) WithGoRand(fn func(r *rand.Rand) error) error {
  	l.Lock()
  	defer l.Unlock()
  	return l.R.WithGoRand(fn)
  }
docs: // WithGoRand invokes the supplied "fn" while holding an exclusive lock
  // for it. This can be used by callers to pull and use a *rand.Rand instance
  // out of the Context safely.
  //
  // The callback's r must not be retained or used outside of the scope of the
  // callback.