def get_addons(objects):
    """Return tuple of required addons for a given sequence of objects."""
    required = {}

    def _required_addons(objs):
        for addon in objs:
            if addon not in required:
                if addon.required_addons:
                    _required_addons(addon.required_addons)
                required[addon] = None

    _required_addons(objects)
    return tuple(required)