Used to filter the output of RelativeStep.
Atomic values are passed though as-is, while node values are sorted
by document order and duplicates removed. An exception is thrown
if there is a mix of atoms and nodes.
Informally: E1/E2 is implemented as:
RelativeStepFilter(for $dot in E1 return E2).
Consume a single position pair.
This PositionConsumer may assume the sequence does no reference
management; i.e. that copyPos is trivial and releasePos is
a no-op. If that is not the case, use consume(TreePosition) instead.