def METHOD_NAME(label_item: Item) -> Optional[Item]:
    """Find link in the item that has "rel" == "source" and return its
    "target" item. If no such link, return None. If multiple such links,
    raise an exception."""
    links = [link for link in label_item.links if link.rel.lower() == 'source']
    if len(links) == 0:
        return None
    elif len(links) > 1:
        raise NotImplementedError()
    image_item = links[0].resolve_stac_object().target
    return image_item