// Add the cursor to the end of the heap.
	heap = append(heap, bc)

	// Sift the new entry up to the right place.
	heapSiftUp(heap, len(heap)-1)
	return heap
}

func heapUpdate(heap []*batchCursor, i int) {
	// Try to sift up.
	if heapSiftUp(heap, i) != i {
		return
	}
	// Try to sift down, if sifting up failed.
	heapSiftDown(heap, i)
}