DNEG-IMITATION is a flag or parameter.
Determine when to produce imitation terms that contain
double negations. Only applies in UN88 when REDUCE-DOUBLE-NEG is T
(in UN88 otherwise, it is implicitly set to ALWAYS; in UN90 it is
implicitly set to CONST-FLEX).
When TPS mates two flexible literals f and g, it adds (f . ~g) as
a dpair. Because it may really have needed (g . ~f), we allow
imitation terms to contain double negations even if REDUCE-DOUBLE-NEG
is T. The options are as follows:
ALWAYS always allows double negations to be used.
CONST forbids them for dpairs of the form (f . ~G), where G is a
constant, but allows them otherwise.
FLEX forbids them for (f . ~g) if g was created by a double negation
in the first place (this prevents endless cycles), but allows them
CONST-FLEX forbids them in the two cases for CONST and FLEX, but allows
them otherwise.
NEVER forbids them outright.
DNEG-IMITATION takes values of type SYMBOL.
It belongs to subjects UNIFICATION .
Its default value is CONST-FLEX
Its current value is CONST-FLEX.

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