"Think of the system as a kind of office," says Holland. "The bulletin board contains the memos that are to be processed that day, and each rule corresponds to a desk in that office that has responsibility for memos of a given kind. At the beginning of the day, each desk collects the memos for which it is responsible. And at the end of the day, each desk posts the memos that result from its activities." In the morning, of course, the cycle repeats. In addition, he says, some of the memos may be posted by detectors, which keep the system up to date about what's going on in the outside world.
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