Odd use-case; delete all but one package

[Jan Danielsson <jan.m.danielsson%gmail.com@localhost>]

Hello,

   I found an alternative solution, so this is more or less a curiosa
question:  Let's say someone wanted to delete all packages except one
(and its runtime dependencies), how would one go about doing that?

   I had a theory that I could mark a package as "preserve", and then
pass -rk to pkg_delete and it would properly resolve the dependency
tree, but I'm not sure how to mark a package as "preserve".  I assumed
that it was something pkg_admin could do, but perhaps preserve:ness is
more fundamental than that?

-- 
Kind regards,
Jan Danielsson