The AutoCAD .net/ObjectARX APIs have a handy feature called the pick first selection.

Dimension Metal Curves Using a Jig from benkoshy on Vimeo.

I can’t say it any better than what’s said here. It’s masterfully explained and brilliantly clear with .gifs to illustrate the concept. Would highly recommend you take a look, especially if you are writing complex multi-threaded applications.

You might not know this but I have a part time job, at the golf shop. One of my duties involves: stacking the shelves full of products.