Oops, ByronK. I had found a 7D that met the set total rule. When it also fitted into my X / Not X test, I was happy. Never looked for the other two possibilities. Sorry!
I have applied the instruction (per my resolution of the message) to every entry. Apart from the operation that we have found, nothing jumps out at me.
So, Barnacle #64, please don't point me at comments that say it's obvious.
Elap, I have no doubt that your trick is very clever, but at this stage it's a GWIT and unfair. I hope to change my mind!