def head_ner(mention):
    """ Compute named entity tag of a mention's head.

    Args:
        mention (Mention): A mention.

    Returns:
        The tuple ('ner', NER), where NER is the named entity tag of the
        mention's head word. If the mention is not a named entity, NER is
        set to 'NONE'.
    """
    return "ner", mention.attributes["ner"][mention.attributes["head_index"]]