code: func (f *FST) Close() error {
  	if f.f != nil {
  		err := f.f.Close()
  		if err != nil {
  			return err
  		}
  	}
  	f.data = nil
  	f.decoder = nil
  	return nil
  }
docs: // Close will unmap any mmap'd data (if managed by vellum) and it will close
  // the backing file (if managed by vellum).  You MUST call Close() for any
  // FST instance that is created.