> How about using a member function like ``get_mixable()`` const?

``register_mixables`` may register more than one mixables to the holder, and it can be "nested", i.e. an algorithm may have a sub-algorithm that has its own mixables (e.g. anomaly algorithms). So this intrusive interface of ``register_mixables`` is more useful, I think.

> It should not be const because the function modifies object relationship.

Hmm. It seems that, in your opinion, e.g. if we used an alternative interface like ``get_mixable()``, then it could be used to modify object relationships around the algorithm, so it also should not be const. Is it right?