Comparator, based on criteria from provided CQLSortNode.
Compares XML documents and is used for sorting XML documents in
ascending or descending order by given Elements in the documents.
Comparison is based on the text content of the Elements
If the field to sort by is not found in one of the documents
under comparison, the comparison will be done using the empty string.
If there are multiple elements with the same name, the first
occurrence will be used for comparison.
The comparator supports multiple sort fields but only the modifiers
sort order (default ascending) and locale (default is the systems default).
Sorting will be character based (numeric sorting not supported).