Hi,
Mojca: Wasn't in obvious that I was joking? (Even though deep down I feared someone would reply with "here it is”.)
I actually thought about doing just that ;) But it will take more work than is justified for simply an email reply. And it is a nice teaching opportunity, so we plan to get back on this topic during the upcoming ConTeXt meeting in Kalenberg.
Luigi: metapost ,head-body-tail or tail-body-head for the filler, lenght of the snake node programming...not bad. I could be useful when one has numbers, and wants to be sure that nobody prepend or append something.
Actually I had a use case for this some years back when I was attempting to reproduce old-fashioned woodblock print. But that predated most luatex development, and I ended up faking the whole thing using \hboxes. Best wishes, Taco