Soar incorrectly rewrites productions with double negated conditions

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It appears that Soar is being too enthusiastic about rewriting conditions
to get rid of double negations. Consider this test case:

sp {init
   (state <s> ^type state)
-->
   (<s> ^x <x>)
   (<x> ^y v1 ^y v2)}

sp {test
   (state <s> ^x <x>)
   -{(<x> -^y)}
-->
   (write (crlf) |Match|)}

Since the rule "test" as written should match when there does not exist an
x attribute without any children y attributes, I would expect that this
rule should match only once when the agent is executed. However, when the
production is loaded into Soar, it is automatically rewritten as

sp {test
     (state <s> ^x <x>)
     (<x> ^y <y*1>)
     -->
     (write (crlf) |Match|)

and ends up matching twice, one for each y attribute on x. So this rewrite
behavior is incorrect.

An easy hack to fix this behavior is to add an extra test to the negated
condition, like this:

sp {test
   (state <s> ^x <x>)
   -{(<x> -^y -^this-can-never-exist xxxxxx)}
-->
   (write (crlf) |Match|)}

Soar will not distribute the outer negation into the inner two. The
behavior of the rule should be exactly the same as long as the
^this-can-never-exist attribute doesn't appear.

Original issue reported on code.google.com by joseph...@gmail.com on 18 Aug 2009 at 9:56