[luatex] multiple callbacks

[Arthur Reutenauer]

> well, one can create a caller for each type of callback, and many are 
> similar

  Sure, absolutely.

> one can never catch alle errors; for instance a style could mess up the 
> node list which can make luatex crash; one just has to assume that the 
> writer does a decent job

  Of course, but a modest safety net can't harm either.  As David said
in BachoTeX, “[With LuaTeX] you get better tools for shooting yourself
in the foot”, so let's give people tools for healing the foot, too :-)

	Arthur