Definition constructive_ground_epsilon_nat (E : exists n : nat, P n) : nat
  := proj1_sig (constructive_indefinite_ground_description_nat P P_decidable E).Definition constructive_ground_epsilon_spec_nat (E : (exists n, P n)) : P (constructive_ground_epsilon_nat E)
  := proj2_sig (constructive_indefinite_ground_description_nat P P_decidable E).Definition P' (x : nat) : Prop := P (g x).

Lemma P'_decidable : forall n : nat, {P' n} + {~ P' n}.