code: func (tx *txn) Delete(key string) keyval.BytesTxn {
  	tx.ops = append(tx.ops, &api.KVTxnOp{
  		Verb: api.KVDelete,
  		Key:  key,
  	})
  	return tx
  }
docs: // Delete adds a new 'delete' operation to a previously created
  // transaction. If <key> exists in the data store, the associated value
  // will be removed.