Class Decidable (P : Prop) := {
  Decidable_witness : bool;
  Decidable_spec : Decidable_witness = true <-> P
}.

Lemma Decidable_sound : forall P (H : Decidable P),
  Decidable_witness = true -> P.

Lemma Decidable_complete : forall P (H : Decidable P),
  P -> Decidable_witness = true.

Lemma Decidable_sound_alt : forall P (H : Decidable P),
   ~ P -> Decidable_witness = false.

Lemma Decidable_complete_alt : forall P (H : Decidable P),
  Decidable_witness = false -> ~ P.

Definition decide P {H : Decidable P} := Decidable_witness (Decidable:=H).

Ltac _decide_ P H :=
  let b := fresh "b" in
  set (b := decide P) in *;
  assert (H : decide P = b) by reflexivity;
  clearbody b;
  destruct b; [apply Decidable_sound in H|apply Decidable_complete_alt in H].

Tactic Notation "decide" constr(P) "as" ident(H) :=
  _decide_ P H.

Tactic Notation "decide" constr(P) :=
  let H := fresh "H" in _decide_ P H.

Require Import Bool Arith ZArith.

Program Instance Decidable_not {P} `{Decidable P} : Decidable (~ P) := {
  Decidable_witness := negb Decidable_witness
}.

Program Instance Decidable_eq_bool : forall (x y : bool), Decidable (eq x y) := {
  Decidable_witness := Bool.eqb x y
}.

Program Instance Decidable_eq_nat : forall (x y : nat), Decidable (eq x y) := {
  Decidable_witness := Nat.eqb x y
}.

Program Instance Decidable_le_nat : forall (x y : nat), Decidable (x <= y) := {
  Decidable_witness := Nat.leb x y
}.

Program Instance Decidable_eq_Z : forall (x y : Z), Decidable (eq x y) := {
  Decidable_witness := Z.eqb x y
}.