code: func (fb *Firebase) LimitToLast(value int64) *Firebase {
  	c := fb.copy()
  	// explicitly not locking here because no one else can
  	// modify this value before we return it.
  	if value > 0 {
  		c.params.Set(limitToLastParam, strconv.FormatInt(value, 10))
  	} else {
  		c.params.Del(limitToLastParam)
  	}
  	return c
  }
docs: // LimitToLast creates a new Firebase reference with the
  // requested limitToLast configuration.
  //
  // Reference https://firebase.google.com/docs/database/rest/retrieve-data#limit-queries