retained accessory power with 1 relay?

I've seen two different diagrams that work, on how to wire relays to retain accessory power at the head unit.   Today I heard there is a way to wire up a single relay to do this. I'm guessing the use of a couple of diodes, but have been drawing out ways for a good hour now and I can't seem to think of how to do this. Anyone heard of this, know how to, or can you think of and draw up a way to do this? No there's no purpose for me to do this. Just a brain exercise. I've heard it "may" be on the master's test. Any master mecp have any thoughts on this?

How about no relay?John DeRosa (Hotwaterwizard)

Stockton California

When in doubt, try it out !