using expression for manipulating paths and points

while perusing the silverlight.net forums i found a question of "creating half circle in blend" and posted my answer (actually posted the xaml of my answer).

rather than try to articulate it in text, which i started out doing and was failing in my own mind, i decided to do a quick screencast demonstrating what i did.  keep in mind i'm not a vector guy but did it the way i knew how.  i hope that someone can correct me with a super easier way of doing it, but until then i'm fine with my method :-).  essentially i used expression design to do most of the manipulation of creating the elements/images i would use in blend.  i think blend does well for some simple element tasks, but if you want to do some more complex things (not that this is a complex example, just making a point), you might be better served in design.  you can view this screencast by subscribing to my podcast feeds on the left area of my blog.

i hope this helps answer the question more visually of what i did.  any questions or methods of improvement are welcome in the comments.