Thanks for you reply.

> >>>>> On Mon, 26 Jun 1995 23:47:44 -0400,
> 	kenn@remus.rutgers.edu (Ken Nakata) said:
> > [~]$ mount -t union -o -b /usr/src/sys $HOME/sys
> > union: Operation not supported
> 
> I think your $HOME/sys partition is mounted by NFS.

Unfortunately, it's not the case.

> Probably you can use "mount -t null -o union" for sys/arch/XXX/conf
> and sys/arch/XXX/compile in this situation, instead of "mount -t union".

Well, actually, I'd like a private source tree where I can change
whatever I want, while I still want access to the original source tree
at the same time.  Hence "-t union -o -b".

I appreciate any input.

Ken