There's an argument I don't understand in "Sheaves in geometry and logic" by Mac Lane and Moerdijk, that seems a priori easy but I can't see it.
Page 174, diagram (10) (involving the power contravariant functor/internal pullback along $g$ and $g'$), they claim that the bottom left square is "formally a pullback", and I'm not sure to understand why it works. Can anyone clarify this little point ?
Thanks.
PS: sorry that I can't rewrite the whole proof/diagrams for people without the book to get it. I'll try to add some details if you think such questions is not good for this site.