code: func (t *txn) Delete(key string) keyval.BytesTxn {
  	t.updates = append(t.updates, &update{
  		key:   []byte(key),
  		value: nil,
  	})
  	return t
  }
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.