Hi David
The current TeX--XeT model is limited but extending it is easy. To solve this issue, three primitives are implemented in parsitex system 1) \vboxjustification: This is just an integer value which sets the direction of \vbox. I can not remember all the details and I can not bother looking at the change file right now but if an integer bigger than 0 is given all \vbox are justified from right and if an integer less than 0 is given, all \vbox are justified from left. if it is 0, then the direction of \vbox is based on \currentvboxdirection (I can not remember the exact name of the primitive, again I have to look at the change file). 2) \cuurentvboxdirection this is again an integer which sets the direction of the current \vbox. 3) \leftvbox \vbox is justified from left 4) \rightvbox \vbox is justified from right As you can see, I am not a fan of LuaTeX (Omega) model because it is just a hack on top of TeX but parsitex adds bidi support to TeX fundamentally and every single detail is considered. Unlike LuaTeX (Omega) model which is not even tested throughly and has so many bugs, the parsitex system has been used over 20 years and it is bug-free.
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