def sem_class(mention):
    """ Compute semantic class of a mention.

    Args:
        mention (Mention): A mention.

    Returns:
        The tuple ('sem_class', SEM_CLASS), where SEM_CLASS is one of
        'PERSON', 'OBJECT', 'NUMERIC' and 'UNKNOWN'.
    """
    return "sem_class", mention.attributes["semantic_class"]