Decorator for version-dependent fields.
    If get_or_add is True (means get), we return s, self.phantom_value.
    If it is False (means add), we return s.

def phantom_decorate(f, get_or_add):
    """
    Decorator for version-dependent fields.
    If get_or_add is True (means get), we return s, self.phantom_value.
    If it is False (means add), we return s.
    """
    def wrapper(*args):
        self, pkt, s = args[:3]
        if phantom_mode(pkt):
            if get_or_add:
                return s, self.phantom_value
            return s
        return f(*args)
    return wrapper