code: func (tx *Tx) DescendRange(index, lessOrEqual, greaterThan string,
  	iterator func(key, value string) bool) error {
  	return tx.scan(
  		true, true, true, index, lessOrEqual, greaterThan, iterator,
  	)
  }
docs: // DescendRange calls the iterator for every item in the database within
  // the range [lessOrEqual, greaterThan), 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.