(** [negligible_exp] is either none (as in FLX) or Some n such that n <= fexp n. *)
Definition negligible_exp: option Z :=
  match (LPO_Z _ (fun z => Z_le_dec_aux z (fexp z))) with
   | inleft N => Some (proj1_sig N)
   | inright _ => None
 end.Inductive negligible_exp_prop: option Z -> Prop :=
  | negligible_None: (forall n, (fexp n < n)%Z) -> negligible_exp_prop None
  | negligible_Some: forall n, (n <= fexp n)%Z -> negligible_exp_prop (Some n).Inductive negligible_exp_prop: option Z -> Prop :=
  | negligible_None: (forall n, (fexp n < n)%Z) -> negligible_exp_prop None
  | negligible_Some: forall n, (n <= fexp n)%Z -> negligible_exp_prop (Some n).Inductive negligible_exp_prop: option Z -> Prop :=
  | negligible_None: (forall n, (fexp n < n)%Z) -> negligible_exp_prop None
  | negligible_Some: forall n, (n <= fexp n)%Z -> negligible_exp_prop (Some n).

(* Specifications: *)
Lemma negligible_exp_spec: negligible_exp_prop negligible_exp.
Lemma negligible_exp_spec': (negligible_exp = None /\ forall n, (fexp n < n)%Z) \/ exists n, (negligible_exp = Some n /\ (n <= fexp n)%Z).
Lemma fexp_negligible_exp_eq: forall n m, (n <= fexp n)%Z -> (m <= fexp m)%Z -> fexp n = fexp m.