A procedure that implements the "response-header" function.
It is implemented by report an attribute objects.
Document-level attributes are otherwise not valid.
A SeqPosition used to represent a node in (usually) a TreeList.
This is special in that the represented node is the current position
of the SeqPosition - but when passed to a method it is only valid
during that method.
Manages a sequence of node references in document order without duplicates.
All elements are POSITION_PAIR_FOLLOWS elements, which makes operations
simple and efficient.