Refractory inhibition of chunks
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When a chunk is built, it may be able to match immediately with those working
memory elements that participated in its creation.  If the production's
actions include preferences for new objects, the production would immediately
fire and create a preference for a new object, which duplicates the
preference that was the original result of the subgoal.  To avoid this, each
production that is built during chunking is refracted so that it will not
fire on the exact set of working memory elements used to create it.  This
does not prevent a newly learned production from matching other working
memory elements that are present.