The conversion NNFC_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 CONDITIONS
Never fails; on non-Boolean terms it just returns a reflexive theorem.
EXAMPLE
# NNFC_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 conjunctive normal form (CNF), as a prelude
to a proof (rather than refutation) procedure. An otherwise similar conversion
NNF_CONV prefers a `disjunctive' splitting and is better suited for a term
that will later be translated to DNF for refutation.