def is_unknown(val: str) ->bool:
    """
    Returns True if val represents and unknown value
    """
    if not isinstance(val, str):
        raise TypeError
    if not val or val.upper() in ('UNKN',):
        return True
    for char in val:
        if char not in ('/', 'X', '.'):
            break
    else:
        return True
    return False
