TRUTHVALUES-HACK is a flag or parameter.
When this flag is T, leaves of truthvalues will not deepened into
an empty disjunction or an empty conjunction. this allows us to deal with
truthvalues in formulas, especially, higher-order formulas. In order to deal
with truthvalues in definitions, such as NULLSET, the definitions containing
falsehood should be rewritten. Please put new definitions containing falsehood
into truthvalues-hack-updatelist so that they can be rewritten appropriately.
TRUTHVALUES-HACK takes values of type BOOLEAN.
It belongs to subjects MATING-SEARCH ETREES .
Its default value is NIL
Its current value is NIL.

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