private ConfigOverlay applyUnsetUserProp(CommandOperation op, ConfigOverlay overlay) {
   List<String> name = op.getStrs(CommandOperation.ROOT_OBJ);
   if (op.hasError()) return overlay;
   for (String o : name) {
     if (!overlay.getUserProps().containsKey(o)) {
       op.addError(format("No such property ''{0}''", name));
     } else {
       overlay = overlay.unsetUserProperty(o);
     }
   }
   return overlay;
 }