\unset in paper block

[Werner LEMBERG]

I wonder whether it is possible to use \unset in the paper block.  The
use-case is lyluatex, which sets `left-margin'; I want to unset this
value so that lilypond only takes the line width into account to
automatically center it.

In other words, I would like to have

   % from lyluatex
   \paper {
     left-margin = 100\pt
     line-width = 200\pt
   }

   ...

   % user code
   \paper {
     \unset left-margin
   }


    Werner