human
Solve the following python programming problem given the following code:
@ classmethod 
def register(cls, attribute_id = None, flag = None) : 
    def register_attribute(klass) : 
        aid = (klass.ID if (attribute_id is None) else attribute_id) 
        flg = ((klass.FLAG | Attribute.Flag.EXTENDED_LENGTH) if (flag is None) else (flag | Attribute.Flag.EXTENDED_LENGTH)) 
        if ((aid, flg) in cls.registered_attributes) : 
            raise RuntimeError ('only one class can be registered per attribute') 
cls.registered_attributes [(aid, flg)] = klass 
        cls.attributes_known.append (aid) 
        if (klass.FLAG & Attribute.Flag.OPTIONAL) : 
            cls.attributes_optional.append (aid) 
else : 
            cls.attributes_well_know.append (aid) 
return klass 
return <MASK> 
    
    Given the code above, what is a proper replacement for <MASK>? Choose among: attribute_id, cls, flag
assistant
#Here is a proper replacement for <MASK>.
def register_attribute(