Add the lower item IDs for one item object and the objects below them.

def _get_id2lower(id2lower, item_id, item_obj):
    """Add the lower item IDs for one item object and the objects below them."""
    if item_id in id2lower:
        return id2lower[item_id]
    lower_ids = set()
    for lower_obj in item_obj.get_goterms_lower():
        lower_id = lower_obj.item_id
        lower_ids.add(lower_id)
        lower_ids |= _get_id2lower(id2lower, lower_id, lower_obj)
    id2lower[item_id] = lower_ids
    return lower_ids