public final class DocumentOrderIterator
DocumentOrderIterator takes as input an iteration of nodes in any order, and
returns as output an iteration of the same nodes in document order, eliminating
any duplicates. An error occurs if the input sequence contains items that are
the next item, or null if there are no more items. Once a call
on next() has returned null, no further calls should be made. The preferred
action for an iterator if subsequent calls on next() are made is to return
null again, and all implementations within Saxon follow this rule.