* I couldn't find information in the manual about the order of hooks.
   It seems that I have to save the original hook by myself and call it
   before/after, right?

* Is there already some code available that I could use as a starting
   point?

They are very old and the syntax has changed in some lua
commands (and a callback is no longer available), but I
found the following articles very useful when I was
learning how the callbacks work:

https://tug.org/tugboat/tb31-3/tb99isambert.pdf

https://tug.org/TUGboat/tb32-1/tb100isambert.pdf

Javier

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