code: func (tx *Tx) Ascend(index string,
  	iterator func(key, value string) bool) error {
  	return tx.scan(false, false, false, index, "", "", iterator)
  }
docs: // Ascend calls the iterator for every item in the database within the range
  // [first, last], until iterator returns false.
  // When an index is provided, the results will be ordered by the item values
  // as specified by the less() function of the defined index.
  // When an index is not provided, the results will be ordered by the item key.
  // An invalid index will return an error.