Definition cnf_normalise {T: Type} (t: Formula C) (tg: T) : cnf NFormula T :=
let f := normalise t in
if check_inconsistent f then cnf_ff _ _
else cnf_of_list (xnormalise f) tg.
Definition cnf_negate {T: Type} (t: Formula C) (tg: T) : cnf NFormula T :=
let f := normalise t in
if check_inconsistent f then cnf_tt _ _
else cnf_of_list (xnegate f) tg.

Lemma xnegate_correct : forall env f,
(make_conj (fun x => eval_nformula env x -> False) (xnegate f)) <-> ~ eval_nformula env f.