code: func SetRand(c context.Context, r Rand) context.Context {
  	if r != nil {
  		r = wrapLocking(r)
  	}
  	return context.WithValue(c, &key, r)
  }
docs: // SetRand sets the current Rand object in the context.
  //
  // Useful for testing with a quick mock. The supplied Rand will be wrapped
  // in a *Locking if necessary such that when it is returned from Get, it is
  // safe for concurrent use.