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\DOC NNF_CONV
\TYPE {NNF_CONV : conv}
\SYNOPSIS
Convert a term to negation normal form.
\DESCRIBE
The conversion {NNF_CONV} proves a term equal to an equivalent in `negation
normal form' (NNF). This means that other propositional connectives are
eliminated in favour of conjunction (`{/\}'), disjunction (`{\/}') and negation
(`{~}'), and the negations are pushed down to the level of atomic formulas,
also through universal and existential quantifiers, with double negations
eliminated.
\FAILURE
Never fails; on non-Boolean terms it just returns a reflexive theorem.
\EXAMPLE
{
# NNF_CONV `(!x. p(x) <=> q(x)) ==> ~ ?y. p(y) /\ ~q(y)`;;
Warning: inventing type variables
val it : thm =
|- (!x. p x <=> q x) ==> ~(?y. p y /\ ~q y) <=>
(?x. p x /\ ~q x \/ ~p x /\ q x) \/ (!y. ~p y \/ q y)
}
\USES
Mostly useful as a prelude to automated proof procedures, but users may
sometimes find it useful.
\COMMENTS
A toplevel equivalence {p <=> q} is converted to {(p /\ q) \/ (~p /\ ~q)}. In
general this ``splitting'' of equivalences is done with the expectation that
the final formula may be put into disjunctive normal form (DNF), as a prelude
to a refutation procedure. An otherwise similar conversion {NNFC_CONV} prefers
a `conjunctive' splitting and is better suited for a term that will later be
translated to CNF.
\SEEALSO
GEN_NNF_CONV, NNFC_CONV.
\ENDDOC
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