Let me re-ask my earlier question because it got overlooked because of the discussion that sparked here directly afterwards 
:
How can we properly migrate to this new way of doing things, while for the time being keeping our code backwards-compatible with older Elixir versions?Use @optional_callbacks but also write defoverridable for backwards compatibility? Or is there a way to switch between these statements at compile-time based on the Elixir version, to make it very explicit that part of it is legacy-support that might be removed in the future?
