Help with my first extension

[illusory]

I'm creating my first Mercurial extension (I'm also a Python newbie), and I've
become stuck.

The extension I'm creating is called 'checkpoint'.  It is very simple, and it
allows the user to create temporary checkpoints when desired.  Here is how it
will work:

    http://paste.pocoo.org/show/RkAbiX8w9pxOVJ9ClSVD/

Here is the code so far:

    http://paste.pocoo.org/show/vNJPtXPmhWgyZIgY1fTX/

My problem right now is lines 24 and 25.  There, I'm attempting to delete the top
changeset (only the top for now, but when the extension is working all checkpoint
changesets will be deleted) and keep local changes.  The problem is that the
"repair.strip" line is causing my working directory to update to revision -1.  Here is
the interaction withMercurial:

    http://paste.pocoo.org/show/Ly90GIfMUvE5TEOVdR1G/

How do I stop this unknown-working-parent problem from happening?

Any assistance would be appreciated.