Goal:
  
  A : Type
  P : A → A → Type
  idx0 : list A
  idx : list A
  a : OnOne2 P (pr1 {| pr1 := idx0; pr2 := idx |})
        (pr2 {| pr1 := idx0; pr2 := idx |})
  ============================
   (NoConfusion_OnOne2 A P
      {| pr1 := {| pr1 := idx0; pr2 := idx |}; pr2 := a |}
      {| pr1 := {| pr1 := idx0; pr2 := idx |}; pr2 := a |})


DEBUG:Going to execute:
match type of id with
           | sigma ?P =>
               let idx := fresh "idx" in
               destruct id as [idx id]; repeat CoreTactics.destruct_sigma idx;
                simpl in id
           | _ => idtac
           end