Definition xnnormalise (t : Formula Z) : NFormula Z :=
  let (lhs,o,rhs) := t in
  let lhs := normZ lhs in
  let rhs := normZ rhs in
  match o with
  | OpEq  => (psub rhs lhs,  Equal)
  | OpNEq => (psub rhs lhs,  NonEqual)
  | OpGt  => (psub lhs rhs,  Strict)
  | OpLt  => (psub rhs lhs,  Strict)
  | OpGe  => (psub lhs rhs,  NonStrict)
  | OpLe =>  (psub rhs lhs,  NonStrict)
  end.

Lemma xnnormalise_correct : forall env f, eval_nformula env (xnnormalise f) <-> Zeval_formula env Tauto.isProp f.