A metaclass, when it is the right tool

Implement class FrozenMeta(type) enforcing that instances only ever set the attributes DECLARED in the class body annotations: - setting an undeclared attribute raises AttributeError at ANY time (init or later) - declared attributes are freely settable and readable - Vec.brand must equal 'frozen' on every class using the metaclass (the metaclass itself may set class attributes) - this must apply to subclasses too — that is why it is a metaclass, not __init_subclass__