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After every move, the system must show the new distribution. It is therefore
an example of the informative function, in the mode autonomously.
Once the player has made a move, it is expected that the system will
think about the available alternatives to achieve its objective (using the cur-
rent state of the game and the knowledge the system may have) and that,
after a while, it will make its own move. In making the move, the system
changes the state of the domain. Therefore, this is a complex example of the
active function.
If the system were naive enough to offer sincere advice on the next
move to the player, that would be an example of the informative function, in
the mode on request.
Example 2.2
Let us consider an e-mail system. The domain consists of users, who can send
or receive messages, distribution lists, active messages, folders created by
users to organize their active messages, and so on. Each message has a con-
tent, a subject, a sender, a date, and one or more receivers. Normally, the
content and the subject of a message are uninterpreted data for the system.
The memory function consists of maintaining a representation of the
state of the domain. The main part of the state will be represented only
within the system, and it is not directly observable in the domain. The state
changes when a user issues a message, receives a message, creates or deletes a
folder, puts a message in a folder (or removes a message from it), and so on.
Among other things, the informative function allows users to visualize
their active messages (at different levels of detail), as well as the contents of
their folders.
The active function consists of sending messages issued by users to their
receivers. The effect is that the sent message is put in the input folder of each
receiver. This function is performed in the mode on request.
Example 2.3
This last example is not a concrete system, but a class of systems: real-time
systems. There is not a consensus on what real-time systems are, but they
tend to be identified by means of a set of common characteristics [7].
First, a real-time system monitors and controls an environment (i.e., it
issues controlling commands that change the environment). Using our ter-
minology, monitoring the environment is a memory function, and control-
ling it is an active function. Second, real-time systems interact with users
for whom they perform a needed function. Such functions may be either
informative or active. Real-time systems frequently have various sensors and
