def ord_prio(prio):
    """Compute the ordinal number of a text priority
    :param prio: string
    :rtype: integer
    """

    return { 'urgmust': 1,
             'must'   : 2,
             'high'   : 3,
             'medium' : 4,
             'low'    : 5 }.get(prio, 5)