code: func (snap *Snapshot) Has(key []byte, ro *opt.ReadOptions) (ret bool, err error) {
  	err = snap.db.ok()
  	if err != nil {
  		return
  	}
  	snap.mu.RLock()
  	defer snap.mu.RUnlock()
  	if snap.released {
  		err = ErrSnapshotReleased
  		return
  	}
  	return snap.db.has(nil, nil, key, snap.elem.seq, ro)
  }
docs: // Has returns true if the DB does contains the given key.
  //
  // It is safe to modify the contents of the argument after Get returns.