`Xedconfsxp` lens failes when trying to save modified tree

[paluh]

Xendconfsxp lens fails on given test:

  test Xendconfsxp.lns put "(logfile /var/log/xend.log)" after
     set "/logfile/item" "test"
     = "(logfile test)"

but if I use sexpr instead of lns it passes:

 test Xendconfsxp.sexpr put "(logfile /var/log/xend.log)" after
   set "/logfile/item" "test"
   = "(logfile test)"

I'm not able to fix this lens - I've tried but I was only able to ''fix'' it by recursion removal.