class ACBMeta(type):

    def __instancecheck__(cls, inst):
        """Implement isinstance(inst, cls)."""
        return any(cls.__subclasscheck__(c)
                   for c in {type(inst), inst.__class__})

    def __subclasscheck__(cls, sub):
        """Implement issubclass(sub, cls)."""
        candidates = cls.__dict__.get("__subclass__", set()) | {cls}
        return any(c in candidates for c in sub.mro())

class Sequence(metaclass=ACBMeta):
    __subclass__ = {list, tuple}

assert issubclass(list, Sequence)
assert issubclass(tuple, Sequence)

class AppendableSequence(Sequence):
    __subclass__ = {list}

assert issubclass(list, AppendableSequence)
assert isinstance([], AppendableSequence)

assert not issubclass(tuple, AppendableSequence)
assert not isinstance((), AppendableSequence)