Parallel-processing version of the ItemMappingIterator class, which executes a mapping function
for each item in a sequence. In this implementation (which is designed initially for use
by the collection() function), the order of items in the result sequence retains
the order of items in the input sequence.
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.
Close the iterator. This indicates to the supplier of the data that the client
does not require any more items to be delivered by the iterator. This may enable the
supplier to release resources. After calling close(), no further calls on the
iterator should be made; if further calls are made, the effect of such calls is undefined.
(Currently, closing an iterator is important only when the data is being "pushed" in
another thread. Closing the iterator terminates that thread and means that it needs to do
no additional work. Indeed, failing to close the iterator may cause the push thread to hang
waiting for the buffer to be emptied.)