Returns a queue-supplied index for item, which is added to the queue. The inserted item follows any existing item with index index in the queue ordering. If index is .nil, item is inserted at the head of the queue. If you omit index, item becomes the last item in the queue.

Inserting an item in the queue at position index will cause the items in the queue after position index to have their indexes modified by the queue object. If inserting an object causes the queue to grow beyond the size, the last item in the queue will be removed.