From Undecidability.Synthetic Require Import DecidabilityFacts SemiDecidabilityFacts.
From Undecidability.Shared Require Import embed_nat.
Local Notation "'if!' x 'is' p 'then' a 'else' b" := (match x with p => a | _ => b end) (at level 0, p pattern).
Definition enumerator__T' X f := forall x : X, exists n : nat, f n = Some x.
Notation enumerator__T f X := (enumerator__T' X f).
Definition enumerable__T X := exists f : nat -> option X, enumerator__T f X.
(* Type enumerability facts  *)
Definition nat_enum (n : nat) := Some n.
Definition unit_enum (n : nat) := Some tt.

Lemma enumerator__T_unit :
enumerator__T unit_enum unit.