Default evaluations
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In the evaluation subgoal, evaluations are usually computed by
domain-dependent productions.  However, sometimes an evaluation can be
computed automatically, based on activity below the subgoal.

     
  1. If an operator is found to be on the path to a solution generated by
     successive lookaheads (success, partial-success, or required-success is
     detected for the desired state), then the operator is given an
     evaluation of partial-success, which will in turn be translated into a
     best preference.
     
  2. If an operator is being evaluated using lookahead but the lookahead
     reaches a dead end (where either no operators are proposed for the
     current lookahead state, or all operators are rejected or prohibited),
     then the evaluation object is augmented with ^symbolic-value failure
     which is translated into a worst preference.