Hi luatex team, mlist_to_hlist callback requires to return a hlist. And that is a big problem because there is no possibility to use this callback if some style already used it. If two different styles uses this callback luatex always throws error "! This can't happen (mlist1)." because mlist is already converted to hlist for second callback. I want to offer a new callback "mlist_to_mlist" were one could access math list without conversion to hlist. Usually if one uses this callback he needs mlist and not a hlist. And probably would be even better to have one more callback "mhpack_filter" which could access result of node.milst_to_hlist(math list converted hlist). So mlist_to_hlist callback would be splited into to parts like: m = list_of_mlist_to_mlist_callbacks (h) n = node.mlis_to_hlist(m) x = list_of_mhpack_filter_callbacks (n) return x What do you think? Linas