ADD-TRUTH : FLAGADD-TRUTH is a flag or parameter.
When set to IF-NEEDED, tests whether the etree has any path of
length 1; if it does, then adds a conjunct TRUTH to the vpform.
When set to T, it will always add this conjunct.
When set to NIL, it will never add this conjunct.
(When TRUTHVALUES-HACK is NIL, it will also add a conjunct NOT FALSEHOOD).
ADD-TRUTH takes values of type SYMBOL.
It belongs to subjects MATING-SEARCH ETREES .
Its default value is IF-NEEDED
Its current value is IF-NEEDED.
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