I'm wondering if there is some good mechanism to test multiple
versions of the same software on the same machine.  For example, I'd
like to be able to test both Emacs 20.7 and Emacs 21.1 on the same
machine.  Further, it would be nice not to have to modify any of the
packages just to be able to do this -- i.e. it'd be nice to have a
general mechanism, not one that's specific to a particular package in
pkgsrc.

After some thinking, it occurred to me that I might be able to prepare
a "base" of packages (ones that don't need to have multiple versions
tested) in /usr/pkg, and then create a separate union mount for each
version of a piece of software I wanted to test.  For example, I'd
perform one union mount to test Emacs 20.7, and I'd unmount that and
union mount something separate when testing Emacs 21.1.

One obvious problem with this is the content of /var/db/pkg.  I
suppose I could also do union mounting for /var/db/pkg, but that seems
error-prone.  Another idea might be to make /var/db/pkg a symbolic
link pointing to some place under /usr/pkg (/usr/pkg/pkg?).  Better
ideas welcome!

There may well be other problems I haven't thought of too ;-)

Does anyone have a better general approach?  If something like this
is documented somewhere already, please let me know where that is.

Any feedback would be much appreciated.