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The Growth of Competence 185

Table 6.1. Goal-situation-action rules that implement the plan in Figure 6.2.

Goal Situation Actions
Go to Nth floor via Standing by elevator door & Set goal to “Call elevator”
the elevator elevator is at some other floor &
call button is not lit
Call elevator Standing by elevator door & push call button
elevator is at some other floor &
call button is not lit
Call elevator Standing by elevator door & Wait
elevator is at some other floor &
call button is lit
Call elevator Standing by elevator door & Enter elevator; goal is sat-
elevator is at the current floor & isfied; set goal to “Use
elevator door is open elevator”
Use elevator is inside elevator & floor button Set goal to “Select floor”
is not lit
Select floor is inside elevator & push button for Nth floor;
button for Nth floor is not lit goal is satisfied; set goal to
& button is within reach “Ride to Nth floor”
Select floor is inside elevator & Ask a person to push the
button for Nth floor is not lit button; goal is satisfied; set
& button is not within reach goal to “Ride to Nth floor”
Ride to floor is inside elevator & elevator is Wait
moving & button for Nth floor
is lit
Ride to floor is inside elevator & elevator is at Exit the elevator; goal is
the Nth floor & elevator door satisfied
is open
Go to Nth floor via is on Nth floor & outside elevator Goal is satisfied
the elevator

action of the selected rule performed. Each turn through this perceive- decide-
act cycle results in some change in the current situation, and the cycle repeats.
A task performance normally consists of a long sequence of perceive-decide-
act cycles, each cycle ending with either a motor action or a cognitive operation
such as an inference or the setting of a subgoal. in programming terminology,
execution is iterative.
Because practical knowledge is hierarchically organized, execution is
also recursive. That is, the top goal is unpacked into a set of subgoals and the
latter are processed and unpacked in turn by the same processes. Eventually,
the process of unpacking encounters elementary actions, which are carried