Definition xnegate (f:NFormula Z) : list (NFormula Z)  :=
  let (e,o) := f in
    match o with
      | Equal  => (e,Equal) :: nil
      | NonEqual => (psub e (Pc 1),NonStrict) :: (psub (Pc (-1)) e, NonStrict) :: nil
      | NonStrict => (e,NonStrict)::nil
      | Strict    => (psub e (Pc 1),NonStrict)::nil
    end.Definition negate {T : Type} (t:Formula Z) (tg:T) : cnf (NFormula Z) T :=
  let f := xnnormalise t in
  if Zunsat f then cnf_tt _ _
  else cnf_of_list tg (xnegate f).

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