def validate_cfop(cfop):
    """Validates C.F.O.P. code

    Valid C.F.O.P. format: '9.999', where 9 is any digit in 0-9.
    """
    if not isinstance(cfop, str):
        return False
    if not '.' in cfop:
        return False
    first_part, last_part = cfop.split('.')
    if not first_part.isdigit() or not last_part.isdigit():
        return False
    if not len(first_part) == 1 or not len(last_part) == 3:
        return False
    return True
