The semantics of productions
----------------------------

Preferences created by the actions of productions are maintained in
preference memory as long as they receive *support*.  Soar distinguishes two
different roles for its long-term knowledge, each of which provides different
types of support for preferences.

One role for long-term knowledge is to retrieve data relevant to the current
situation.  For example, productions that propose or control the selection of
problem spaces, initial states, and operators all retrieve structures from
long-term memory that are relevant to the current context.  If the current
context changes such that a production no longer matches, the preferences
created by the production will not be relevant.  Similarly, productions that
refine the current problem space, state, or operator and their sub-objects
fall into this category.  Production instantiations representing this type of
knowledge provide *I-support* for the preferences in their actions as long as
the conditions of the productions are satisfied.  The I-support is removed
when the production no longer matches.  If a preference has no remaining
support following a retraction, it will be removed from preference memory.

The other role for long-term knowledge is to retrieve the changes that should
be made to the current situation based on the current operator and state.
Operator application and operator instantiation through refinement are the
two PSCM operations carried out by these productions.  The actions of these
productions make changes to the context that persist even after the
production is no longer satisfied by providing *O-support* for their actions.
These productions are syntactically distinguished by the fact that they test
the operator and the state in their conditions and they create preferences to
modify either the operator or the state in their actions.  A preference keeps
its O-support until its value is rejected, it becomes unlinked from the
context stack, or the subgoal in which it was created terminates. See Section
See Recognition memory on page See Recognition memory and Section
See Subgoal results on page See Subgoal results for complete details
of the support of preferences.  Appendix See Support of Preferences on
page See Support of Preferences contains a formal description of how
support is calculated.