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Exchange requires both accounts to be idle before we start exchanging (i.e. exchanging a with b while a is still exchanging with c is not allowed)
For transfer, there is no such requirement. Louis is wrong.
In the exchange problem, state of one account depends on the state of another account (they are coupled). There is no such coupling in the transfer problem.
Another way to rephrase what sam says is to say that exchange has an unserialized access (the difference process) which can occur concurrently with another exchange underway while transfer has no such process...