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A two-party exchange exchanges electronic goods between two participants, 0 (for “originator) and R (for “recipient”). We consider three types of electronic goods: confidential data, money (payments), and signatures on public data. In order to start an exchange, each party X (one of 0 and R) has to input the following parameters:

1. item, the item X wants to send’. 2. descr, a description of item,, detailed enough to identify all important properties of the item to the person receiving it. For example, the description of contract can be the text of the contract.

3. expect,(descr,, descr,) a predicate which formalises the expectation of a participant. It evaluates to true if the user X is satisfied when receiving an item described by dcscr, in exchange for an item described by descr,. 4. fits (descr, item) a predicate which evaluates to true if the description fits the item. This predicate cannot be evaluntcd automatically for some types of …