How can I mark a file backed up?

[Greg Fisher <fisher@ctt.bellcore.com>]

I would like to reset the :not-backed-up property of a file so the 
directory will list it as being backed up.  However, the :not-backed-up
property is not a setable-property of a file.  As such, the command
(fs:change-file-properties pathname nil :not-backed-up nil)
has no effect.  The dumper program obviously is able to change this 
flag, but it is not clear how it does it.

Does anyone know how to reset this file property.

Greg Fisher
fisher@ctt.bellcore.com