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>    Most-importantly, when references and (smart) pointers to
>    polymorphic classes are converted to python, we will attempt to
>    downcast to "self_base", and if successful, we'll return the
>    contained self pointer instead of a new object

This suggests that agent is derived from self_base, not multiply-inherited
in agent_callback. The agent might have been created from the c++ side - so
the self_pointer might be null. How do we deal with this case?