Hi, Sorry if this is very clear from the manual, maybe I just didn't read the part where it is explained (or I read it but was too stupid/tired to notice), but I don't understand the meaning of the return value of some callbacks. The first example (by order of apparition in the manual) is pre_linebreak_filter, whose calling convention is: function(<node> head, <string> groupcode) return true | false | <node> newhead end I think it's clear what returning a newhead node does, but I don't understand what returning true or false means for this callback. Other similar callbacks include post_linebreak_filter, (h|v)pack_filter, pre_output_filter, etc. The underlying problem is, I was trying to enhance the user documentation of luatexbase-mcb (which allows to register many functions in a single callback, based on some ConTeXt code I believe) but I can't make sense of the calling conventions in the current code (the functions are expected to return a node *and* true, in whatever order), so that I can't check it is correct, and even less document why it works that way. I guess understanding the calling convention for the callbacks themselves would be a good starting point. Thanks, Manuel.