Negated conditions
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In addition to the positive tests for elements in working memory, conditions
can also test for the absence of patterns.  A condition preceded by a dash
(i.e., "-") is called a *negated condition* and will be satisfied only if
there does not exist a working memory element consistent with its tests and
variable bindings.  Thus it is a test for the absence of a working memory
element.  For example, the following condition tests the absence of a working
memory element augmentation of the object bound to <P1> of type father.

-(<P1> ^type father)

A negation can be used within an object with many attribute-value pairs by
having it precede a specific attribute:

(<P1> ^name john -^type father ^spouse <p2>)

A negation can also be used for a path description, in which case it test for
the complete test of the path.

(<P1> ^name john ^type father -^spouse.sister.name sally)

A negation can also precede an attribute with multiple values.  In this case
it test for the absence of the conjunction of the values.  For example

(<P1> ^name john -^children <c1> <c2>)

tests *(not ((children <c1>) and (children <c2>)))*.  This can be combined
with a path description:

(<P1> ^name john ^type father -^spouse.sister.name sally judy)

which gets expanded to:

  (<P1> ^name john ^type father)
-{(<P1> ^spouse <s*1>)
  (<s*1> ^sister <s*2>)
  (<s*2> ^name sally)
  (<s*2> ^name judy)}

This will match if john does not have at least two sister-in-laws with names
sally and judy: *(not ((spouse.sister sally) and (spouse.sister judy)))*