> Jan Tosovsky <mailto:j.tosovsky@email.cz>
> 31. März 2016 um 21:28
>
> Wow, I am quite satisfied with this solution.
>
> Btw, I couldn't find any details for that \leftboundary command either 
> in TeX [1] or ConTeXt [2] command references.
The \leftboundary command uses a new LuaTeX primitive which means you 
can’t find it in a TeX [1] reference.

spac-ver.mkiv:

\unexpanded\def\leftboundary   {\boundary\plusone}
\unexpanded\def\rightboundary  {\boundary\plustwo}

Wolfgang