Hi,

I've noticed that, in GuixSD, if one rolls back the system with 'guix
system roll-back' or switches to a previous generation with 'guix system
switch-generation', and then one tries to run 'guix system reconfigure',
the operation fails because the symlink for the new generation already
exists.  I expected the operation to succeed and overwrite the existing
generation, since that mirrors the behavior of 'guix package'.

Specifically, if the current generation is N and generation N+1 exists,
then running 'guix system reconfigure' should overwrite generation N+1
with the new system.  Currently, it fails to create the symlink.  I've
attached a patch which modifies the behavior to be what I expected.

-- 
Chris