Improve compatibility with LuaTeX 1.18.0

lua-ul.dtx

@@ -245,15 +245,11 @@ local callback_define
 % HACK: Do not do this at home! We need to define the engine callback directly,
 % so we use the debug library to get the ``real'' \verb+callback.define+:
 %    \begin{macrocode}
-for i=1,5 do
-local name, func = require'debug'.getupvalue(luatexbase.disable_callback, i)
-  if name == 'callback_register' then
-    callback_define = func
-    break
-  end
-end
-if not callback_define then
-  error[[Unable to find callback.define]]
+do
+  local saved_luatexbase, saved_callback_register = luatexbase, callback.register
+  luatexbase.uninstall()
+  callback_define = callback.register
+  luatexbase, callback.register = saved_luatexbase, saved_callback_register
 end
 
 local function filtered_append_to_vlist_filter(box,