Horse Trading
Actually, the German term Kuhhandel would be better for this, as it concerns any transaction, not just difficult ones. The salient feature of any transaction is that it isn't concluded until all involved parties agree. As the transaction progresses, it is always possible for any party to break it off and return to the initial state. But let me start at the beginning of the story.
Recently I used one of the cash checkout machines at a supermarket, as is my wont. The bill came to CHF 8.20, which I dutifully fed into the belly of the beast. After all the clanking was done, it claimed that I still owed one Franc. I called over the attendant, who thoroughly checked the machine, and, at my request, instructed it to regurgitate the money that I had fed in. It dutifully coughed up 7.20, but what was illuminating is that this included a 20-Rappen (cent) coin. I had not fed in a 20-Rappen coin, but two 5-Rappen coins and a 10-Rappen coin. Thus it became clear that it was not giving back the coins that I had fed in; it had simply dipped into its treasury to spit out any old coins that would add up to the sum that it considered correct.
Now at this point a normal person would have concluded that they must have simply mis-counted. I, however, was convinced otherwise, and wrote an email to the manager to complain about my experience. It wasn't until I wrote this email that I realized why this experience made me so uneasy. My complaint was that the machine had no intermediate buffer for all the cash involved in the transaction. Lacking this, when asked to give back the money that I had tendered, it could only give back an equivalent amount, not necessarily the coins and bills that I had given it. At a more fundamental level, it lacks an abort button that would enable the user to undo everything and return to the previous state.
An illustration makes this clear. Let's say that you are buying a cow from me for $1700. As we stand out there in the field, you peel off $100 notes one at a time and hand them to me. I then put each one into my pocket as I get it. After you have passed me 17 of them, you demand your cow. Now I claim that you only gave me $1600, and you'll get your cow as soon as you dish out another $100. At this point you will naturally demand to see the money that you have handed me. I then fish $1600 out of my pocket. You are now exactly at the point where I was in my "transaction" with the cash checkout machine. You call me a liar and a swindler, and we duke it out. Sadly, I couldn't do that with the checkout machine.
This is why, in the real world, we put all the cash on the table so that everyone can count it and agree to the amount. But our sophisticated cash checkout machine can't do this, so what we have isn't a proper transaction. I give my cash until it thinks I've put in enough, and I have no recourse if I think otherwise.
The moral of the story is that if your checkout machine doesn't have an abort button, then the machine is incapable of performing a real transaction, and you should avoid it. It is in charge, and you have simply to meet its demands. This is how machines become our masters instead of our servants.
