As a real world example: Synctus depends on being able to do this correctly (what it does is described at http://www.synctus.com/tech/ http://www.synctus.com/te

As a real world example: Synctus depends on being able to do this correctly (what it does is described at http://www.synctus.com/tech/ http://www.synctus.com/tech/ and the right hand column at http://www.synctus.com/tech/locking.html http://www.synctus.com/tech/locking.html) - consider how the lock transfer might work in the context of the Two Generals' Problem. How exactly I do it is currently a trade secret. I've worked around the problem, but the issues surrounding this are a big part of the solution.