It's ultimately still macro expansion. Don't get me wrong, I think that the work of Frank Mittelbach et al is nothing short of miraculous (and has been for the

It's ultimately still macro expansion. Don't get me wrong, I think that the work of Frank Mittelbach et al is nothing short of miraculous (and has been for the roughly thirty years since Frank burst upon the scene). If you look under the hood it gets clear just how tough programming in TeX macros is. I have a half-finished extension to expl3 to create a zip function that will take two lists and a macro to zip the two lists into a single list (half-finished because the motivating factor was a tex.stackexchange question and there's a similar function that's underdocumented and not a proper zip that I was able to use to provide an answer).