private fun current_iterator: nullable Iterator[E]
	do
		if previous_iterator == null then
			# Get the first sub-iterator.
			if inner.is_ok then
				previous_iterator = inner.item
				previous_iterator.start
				inner.next
			else
				return null
			end
		end
		# Get the first sub-iterator that has a current item.
		while inner.is_ok and not previous_iterator.is_ok do
			previous_iterator.finish
			previous_iterator = inner.item
			previous_iterator.start
			inner.next
		end
		return previous_iterator
	end