code: func (b *FastLogStore) FirstIndex() (uint64, error) {
  	b.mu.Lock()
  	defer b.mu.Unlock()
  	if b.closed {
  		return 0, ErrClosed
  	}
  	if b.limits {
  		return b.min, nil
  	}
  	b.fillLimits()
  	return b.min, nil
  }
docs: // FirstIndex returns the first known index from the Raft log.