I came across a רבינו יואל מבונ"א quoted by Rosh in his commentary to Kiddushin (4:16). See here. Who is he?
I suspect that he is Rabbeinu Yoel HaLevi, father of Ra'avyah and that בונ"א is what we know today as the city of Bonn in Germany.
Can anyone provide a confirmation or a refutation of my suspicions?