code: func (tx *Txn) Delete(key string) keyval.BytesTxn {
  	if tx.addPrefix != nil {
  		key = tx.addPrefix(key)
  	}
  	tx.ops = append(tx.ops, op{key, nil, true})
  	return tx
  }
docs: // Delete adds a new 'delete' operation to a previously created
  // transaction.