Preference processing
=====================

 
In determining changes to working memory, preferences for all augmentations
except context slots are processed as soon as they are created by production
firings during the *preference phase* of the elaboration cycle.  Figure ?
shows an overview of a complete decision cycle as well as details of the
phases of an elaboration cycle.  Based on the preferences, changes are made
to working memory in what is called the {working memory phase} of the
elaboration cycle.  Once changes are made to working memory, the production
memory is tested again for new instantiations or retractions.  The
elaboration cycles continue until *quiescence*, that is, until no more new
production instantiations are created, and no more retract (see Section 
See Adding Input and Output Routines on page *Note Adding Input and Output
Routines:: for details of how I/O fits in with an elaboration cycle).  The
total set of elaboration cycles that occur before quiescence is called Soar's
*elaboration phase*.

Once Soar reaches quiescence, the decisions for context slots are made.  This
process is called the *decision phase*.  The determination of values for
these context objects differs in five ways from other augmentations:

  1. Decisions for context slots are made only during the decision phase.
     
  2. Decisions are made only for context slots that do not have a current
     value, or for context slots for which there is a *reconsider* preference
     for the current value.
     
  3. A state slot can have a value only if a problem space object was
     selected before the current decision phase, and a operator slot can have
     a value only if a state object is selected before the current decision
     phase.
     
  4. If a new problem space is selected, the state and operator context slots
     are eliminated because they are no longer relevant to the new problem
     space.  Similarly, if a new state is selected, the operator context slot
     is eliminated.
     
  5. If a change is made to a context slot, all newer contexts (subgoals) are
     eliminated, as they will no longer be relevant.

The elaboration phase together with decision phase completes the basic cycle
of Soar.  The elaboration phase implements the PSCM functions of proposal,
comparison, and refinement.  The context decisions determine the behavior of
the system and implement the PSCM functions for selecting a problem space,
selecting an initial state, selecting an operator, applying an operator, and
terminating an operator.  Context decisions lead directly to irreversible
actions through changes in the context, through the application of operators
that modify the external world, or through changes to the internal state.
Because the context decisions are delayed until quiescence, all relevant
permanent knowledge has been retrieved when these PSCM functions are
performed.

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Preference semantics