(** We build the subset [A'] of [A] made of [a1] and [a2] *)

Definition A' := @sigT A (fun x => x=a1 \/ x=a2).Definition a1':A'.Definition a2':A'.

Lemma decide : forall x:A', exists y:bool , (projT1 x = a1 /\ y = true ) \/ (projT1 x = a2 /\ y = false).