// Reverse the ancestors relation to get the descendants relation.
Source mktSegmentDescendants =
mktSegment.join(mktSegmentAncestors, mktSegment.value());
// Select an element of the hierarchy.
Source selVal = mktSegment.selectValue(node);
// Select the descendants of the specified element.
Source selValDescendants = mktSegmentDescendants.join(mktSegment,
selVal,Source.COMPARISON_RULE_DESCENDING,
false);