Acceptable preferences
......................

The only preferences that exist in working memory are acceptable preferences
for the context slots (the problem-space, state, and operator augmentations
of goals).  That is, other preferences cannot be examined by matching them
with conditions.  Acceptable preferences for problem spaces, states, and
operators can be matched in a condition by testing for a "+" following the
value.  This allows a production to test the existence of a candidate object
and its properties, and possibly create a preference for it, before it is
selected for a context slot. In the example below, {^$operator <o> +} matches
the acceptable preference for the operator augmentation of the goal, while
^$state <s> tests that state {<s>} has been selected as the current state for
the goal.

(sp blocks*example-production-conditions
  (goal ^problem-space <p> ^state <s> ^operator <o> +)
  (<p> ^name blocks-world) 
  (<s> ^table <t1> ^ontop <c1> { <> <c1> <c2> })
  (<t1> ^color << red blue >> )
  (<o> ^name move-block)
  ...
  -->
  ...
)