@classmethod
def is_incompatible_class(manager, cls):
    """ is_incompatible_class(cls)
        
        Returns a string giving the reason why the given class
        if not SSDF-compatible. If the class is compatible, this
        function returns None.
        
        """
    if not hasattr(cls, '__to_ssdf__'):
        return "class does not have '__to_ssdf__' method"
    if not hasattr(cls, '__from_ssdf__'):
        return "class does not have '__from_ssdf__' classmethod"
    if not isinstance(cls, type):
        return 'class is not a type (does not inherit object on Python 2.x)'
