What is a reference for ["the equivalence between geometric theories and frames internal to the free topos"](https://indico.math.cnrs.fr/event/747/contributions/2523/attachments/1818/1962/introductionJoyal.pdf)? [1] This seems to be an extremely interesting theorem.

[1] André Joyal, “A crash course in topos theory: the big picture”, lectures at Topos à l'IHÉS, 2015.