First off, I've been playing with Spring Rich for a couple of weeks now and I'm impressed with what I'm seeing - got a sample application up and running, integrated with a couple of remote services, much quicker than I could have previously. Thanks a lot, great work.

Now my question. I'm trying to add in some validation but I can't figure out how to implement a specific situation: I have one form with five text fields. My validation is that one of four of them is required, but not more than one. The final text field is optional, but not sufficient.

Anyone have any pointers on how to implement this as a Rules implementation?