Impasses and subgoals
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Impasses can arise for all augmentations.  There are four types of impasses.
Three of these can arise for any augmentation when the available preferences
for an attribute either are insufficient or conflicting (tie, conflict, and
constraint failure).  The remaining impasse, no-change, arises only when none
of the context slots are changed during the decision procedure.  This impasse
arises from the inability to make progress with any of the contexts in the
context stack.  Below is a short description of the four types of impasses.

  1. *tie*: when there is a collection of equally eligible objects competing for
     a particular context slot;
     
  2. *conflict*: when two or more objects are better than each other, and they
     are not dominated by a third object;
     
  3. *constraint-failure*: when there are conflicting necessity preferences;
     
  4. *no-change*: when the elaboration phase runs to quiescence without 
     suggesting any change to the context.

An impasse is resolved whenever a decision can be made for the corresponding
attribute.  For example, if there is a tie impasse between two objects, the
impasse will be resolved when a new preference is added to working memory
that prohibits one of the choices, rejects one of the choices, requires one
of the choices, makes one a best choice, makes one better than another, makes
one a worst choice, or makes them both indifferent.  If there is a tie
between three objects, the tie will be broken when one of the objects (or a
set of indifferent objects) dominates the others.  So the impasse will be
resolved if a best or require preference is created for one of the objects,
if one object is made better than the other two, and so on.

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Goal and impasse object augmentations