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


DEBUG:Going to execute:
fun id =>
             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