code: func (tr *Transaction) Has(key []byte, ro *opt.ReadOptions) (bool, error) {
  	tr.lk.RLock()
  	defer tr.lk.RUnlock()
  	if tr.closed {
  		return false, errTransactionDone
  	}
  	return tr.db.has(tr.mem.DB, tr.tables, key, tr.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 Has returns.