>So the contract is a deposit ATM where you can deposit cash and it spits out a bank check, but if you deposit a bank check it just shreds the check.

>Or a currency conversion machine where you can shove USDs in the slot to get EURs, but if you shove EURs in, to the shredder they go,

Very good explanation, thanks, though the machine you describe would not probably be called a "smart" ATM.